I figured I'd throw this out to the fun and sane bunch here

I found out a good friend got dinged with a ticket for having a tinted plate cover. My guess is he probably was going a bit fast and the officer couldn't get a reading...so they went for the plate cover. It's a $110 ticket with no points.
I know that plates have to be visible, but on what criteria for the covers? I've often heard that all covers are technically illegal because they can block the view of the plate.