It's BS but they are crackin hard on riders in the bay area. What did we do to them I think it's just jealousy . Sounds like he was itchin to ticket someone and you lost the raffle so to speak. (Thats what I got told once by LE)
PanadaNin is right this could just be a "fix it" ticket. Here in Idaho you can have a CLEAR plastic lens over your licence plate. You just have to make sure your plate is clear. On the other hand we can get an obstructed plate ticket for having mud and or snow on the plate. Normaly not ticked if you are are not a jerk about it.
(f) A covering shall not be used on license plates except as follows:
(1) The installation of a cover over a lawfully parked vehicle to protect it from the weather and the elements does not constitute a violation of this subdivision. Any peace officer or other regularly salaried employee of a public agency designated to enforce laws, including local ordinances, relating to the parking of vehicles may temporarily remove so much of the cover as is necessary to inspect any license plate, tab, or indicia of registration on a vehicle.
(2) The installation of a license plate security cover is not a violation of this subdivision if the device does not obstruct or impair the recognition of the license plate information, including, but not limited to, the issuing state, license plate number, and registration tabs, and the cover is limited to the area directly over the top of the registration tabs. No portion of a license plate security cover shall rest over the license plate number.
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.
Keep someone from stealing your sticker. Hell a ton of people in Philly won't even leave their tag on the vehicle (car) but keep it in the rear window. I hear about a lot of plate and sticker theft... So there are reasons other than asshatery to cover your plate... I believe he said his plate was clear so it is probably just a revenue generator. But if it is state law to keep it uncovered so be it.