Plate cover vehicles should have their registrations revoked and the owner should not be permitted to register any vehicle for a year.  License plate covers are a bold statement that the driver intends to evade photo-enforcement accountability for bad driving practices that endanger the rest of us.

State Regulations/Prohibitions Concerning License Plate Covers (italics added):

Arizona  California  Delaware  District of Columbia  Illinois  Indiana  Iowa  Kansas  Massachusetts  Missouri   New Jersey    New York   North Carolina  Ohio  South Carolina  Tennessee  Texas  Virginia    (adding more but this is a start and heads-up for vehicles from these states)

Alabama

Mutilated or altered license plates are not to be displayed on any motor vehicle. The terms "mutilation" or "alteration" as used in Section 40-12-265, Code of Alabama 1975, shall include, but shall not be limited to, the following: Any situation wherein an unauthorized type decal, sticker, patch or other device or impediment which would tend to block or obscure the license plate number or the revalidation decals or otherwise make the Alabama license plate or decal illegible.

Arizona

State law, ARS 28-2354(C), requires that the state name at the top of Arizona license plates must be visible. Any license plate holder or cover that prevents the license plate from being clearly legible, is in violation of this law.

California

CAL. VEH. CODE § 5201  License plates shall at all times be securely fastened to the vehicle for which they are issued so as to prevent the plates from swinging, shall be mounted in a position so as to be clearly visible, and shall be maintained in a condition so as to be clearly legible.  (g)A casing, shield, frame, border, product, or other device that obstructs or impairs the reading or recognition of a license plate by an electronic device operated by state or local law enforcement, an electronic device operated in connection with a toll road, high-occupancy toll lane, toll bridge, or other toll facility, or a remote emission sensing device, as specified in Sections 44081 and 44081.6 of the Health and Safety Code, shall not be installed on, or affixed to, a vehicle.

 Delaware

Delaware Code Section 2126  (c) No number plate, or any portion thereof, shall be covered with any tinted material, nor shall any other material be placed on or around a number plate which would conceal and/or obscure any information contained thereon, including the registration expiration sticker. Plate frames that do not conceal and/or obscure any information contained on the plate, including the registration expiration sticker, are not prohibited by this section.   (e) It shall be unlawful to sell, offer to sell, transfer, possess or use any kind of device, product, plate cover, or object, including any image altering device or spray, for the purpose of hindering, inhibiting, impeding, impairing, or preventing the photographing, recording or imaging of a license plate in connection with the enforcement of this motor vehicle code or any local or municipal traffic laws.

District of Columbia

District law requires that the tags are to be visible, and may not be obstructed or covered with any substance (e.g. glass, plastic, spray, license tag holders, etc.)

 Illinois

In 2007, Illinois Senate Bill 1265 was passed, which clarified that any vehicle license plate cover used by a driver in Illinois was illegal. License plate covers were already banned in the state's vehicle code, although lawmakers passed Senate Bill 1265 to clarify a contradictory piece of code and ensure that clear plastic covers were outlawed along with tinted covers

 Indiana

IC 9-18-2-26 License plates; display  Sec. 26. (b) A license plate shall be securely fastened, in a horizontal position, to the vehicle for which the plate is issued:
(3) in a place and position that are clearly visible;
(4) maintained free from foreign materials and in a condition to be clearly legible; and
(5) not obstructed or obscured by tires, bumpers, accessories, or other opaque objects

Iowa

321.37 Display  It is unlawful for the owner of a vehicle to place any frame around or over the registration plate which does not permit full view of all numerals and letters printed on the registration plate.

Kansas

8-15,110. Covering license plate with plastic-like material. (a) It shall be unlawful for any person to attach and display on any vehicle a license plate, as required under article 1 of chapter 8 of the Kansas Statutes Annotated, which is covered, in whole or in part, with any clear or opaque material or any other plastic-like material that affects the plate's visibility or reflectivity.

 Massachusetts:

540 Code of Massachusetts Regulations (CMR) 2.23: Display of Reflectorized License Plates  (1) On or after August 1, 1969, no reflectorized number plate issued by the Registrar of Motor Vehicles and mounted on any motor vehicle or trailer shall be covered with any glass, plastic or similar material if such material reduces the legibility or substantially diminishes the reflective qualities of such plate.  (2) Any such number plate covered with any glass, plastic or similar material which reduces the legibility or substantially diminished the reflective qualities of such plate shall be deemed not to be maintained in good order, and in violation of the provision of said M.G.L. c. 90.  (3) Nothing contained in 540 CMR 2.00 shall be construed to prohibit the use of any metal or other frame covering, the border of any such reflectorized number plate so long as such frame does not cover or obscure in any manner the register number or any other words, symbols or numbers lawfully imprinted on or affixed to such number plate.

 Missouri

LICENSE (REGISTRATION) PLATES — Missouri law requires that license plates be securely fastened to a vehicle, and that all parts of the plates are visible and clean for view or inspection by any law enforcement entity. If you have frames around your license plates, the Missouri State Highway Patrol reminds you that the frames must not cover any part of the plates.

 New Jersey:

(N.J.S.A. 39:3-33) Motorists will receive two matching license plates upon registering a vehicle. One plate is provided when registering a trailer, moped or motorcycle. For passenger vehicles, one plate should be attached to the front of the vehicle and the other to the rear at least 12 inches but less than 48 inches above the ground. Both plates must be clean and visible. The rear plate must be lighted so it is visible from 50 feet at night, even with reflectorized plates (N.J.S.A. 39:3-48b). Using license plate covers or holders that obscure or conceal aconceal any lettering on the license plate is a violation, with a fine of up to $100.

New York State:

Article 14 - REGISTRATION OF MOTOR VEHICLES  § 402. Distinctive  number; form of number plates; trailers.

1.(b)  Number  plates shall be kept clean and in a condition so as to be easily readable and shall  not  be  covered  by  glass  or  any  plastic material,  and  shall  not  be  knowingly  covered  or  coated  with any artificial or synthetic material or substance that conceals or  obscures such  number plates or that distorts a recorded or photographic image of such number plates, and the view of such  number  plates  shall  not  be obstructed  by  any  part of the vehicle or by anything carried thereon, except for a receiver-transmitter issued by  a  publicly  owned  tolling facility  in  connection  with  electronic  toll  collection  when  such receiver-transmitter  is  affixed  to  the  exterior  of  a  vehicle  in accordance with mounting instructions provided by the tolling facility.

 North Carolina

General Statutes 3. (g) … Any operator of a motor vehicle who shall willfully cover or cause to be covered any part or portion of a registration plate or the figures or letters thereon by any device designed or intended to prevent or interfere with the taking of a clear photograph of a registration plate by a traffic control or toll collection system using cameras commits an infraction and shal and shall be penalized under G.S. 14-3.1.

 Ohio

4503.21 (A) …. All license plates shall be securely fastened so as not to swing, and shall not be covered by any material that obstructs their visibility.

 South Carolina

§ 56-3-1240  Every license plate, at all times, must be fastened securely in a horizontal and upright position to the vehicle for which it was issued so as to prevent the plate from swinging. However, if a motorcycle is equipped with vertically mounted license plate brackets, its license plate must be mounted vertically with its top fastened along the right vertical edge. The bottom of the plate must be at a height of not less than twelve inches from the ground in a place and position clearly visible as provided in Section 56-5-4530, and it must be maintained free from foreign materials and in a clearly legible condition. No other license plate, lighting equipment, except as permitted in Section 56-5-4530, tag, sign, monogram, tinted cover, or inscription of metal or other material may be displayed above, around, or upon the plate other than that which is authorized and issued by the Department of Motor Vehicles for the purpose of validating the plate.

 Tennessee

§ 55-4-110. (b) Every registration plate shall at all times be …. in a place and position to be clearly visible and shall be maintained free from foreign materials and in a condition to be clearly legible. No tinted materials may be placed over a license plate even if the information upon the license plate is not concealed.

 Texas

TEX TN. CODE ANN. § 502.409 :(a) A person commits an offense if the person attaches to or displays on a motor vehicle a number plate or registration insignia that:   (5) has blurring or reflective matter that significantly impairs the readability of the name of the state in which the vehicle is registered or the letters or numbers of the license plate number at any time;  (6) has an attached illuminated device or sticker, decal, emblem, or other insignia that is not authorized by law and that interferes with the readability of the letters or numbers of the license plate number or the name of the state in which the vehicle is registered; or  (7) has a coating, covering, protective material, or other apparatus that: (A) distorts angular visibility or detectability; (B) alters or obscures one- half or more of the name of the state in which the vehicle is registered; or (C) alters or obscures the letters or numbers of the license plate number or the color of the plate.

  Virginia:
Code: § 46.2-716. B. No colored glass, colored plastic, bracket, holder, mounting, frame, or any other type of covering shall be placed, mounted, or installed on, around, or over any license plates if such glass, plastic, bracket, holder, mounting, frame, or other type of covering in any way alters or obscures (i) the alpha-numeric information, (ii) the color of the license plates, (iii) the name or abbreviated name of the state wherein the vehicle is registered, or (iv) any character or characters, decal, stamp, or other device indicating the month or year in which the vehicle's registration expires. No insignia, emblems, or trailer hitches or couplings shall be mounted in such a way as to hide or obscure any portion of the license plates or render any portion of the license plates illegible.