First, if you look at the package when you buy those license plate covers, it says something to the effect of "Not legal in all states. Check your local laws..." As far as why they sell them? Because people keep buying them.
Dave beat me to the TCA, but I will reemphasize one part of it - "No tinted materials may be placed over a license plate even if the information upon the license plate is not concealed."
Just depends on the officer. We all have the tint meters. Personally, I won't mess with anyone except for the cars whose windows look like they're painted black. Some guys enforce it like it's going out of style. Others have never written a ticket for it.
There are a LOT of cops who won't mess with window tint or tinted license plate covers. Still doesn't make it right. I may not stop people for speeding until they hit 11 or 12 over the limit, but the people doing 10 over are still speeding.