In the US there are often noise ordinances and laws that largely go unenforced. Step 1 is enforcing existing laws.
Seems like the illegal covers would be straightforward to enforce any time these cars are parked on public streets.
It also adds a bit of deterrence against tag theft, depending on how you secure it.
A person walking by may not be able to tell the difference between one meant to protect the plate and one that hides the plate from cameras, which can cut both ways. I've heard from people who have non-blocking plates get tickets because they were assumed to be the camera-defeating variety, even though they were basic polycarbonate covers.