This cover sounds like something that could easily be illegal depending on the location.

Quick research says that license plate covers are "Allowed" in just 16/50 states, though I have no idea how accurate the source is and it also says that the "Allowed" designation "only" means that that that state requires license plates to be easily readable. (One of the table rows is "Arizona: Arizona | Arizona", so perhaps it's not very accurate.)

Still, regardless of how reliable the sources I found are, I'd be shocked to find many states that don't have prohibitions on making it difficult for red-light/speed/ALPR cameras to read plates.