Well, the Lambo is a supercar, and the Cayenne is just ugly anyway so it wouldn't matter
Actually in some states here in the US the covers are illegal. Why? Because they prevent speed cameras and red-light beater cameras from effectively taking photographs of the license plates (which are reflective). At least that's how it is in California. But that's been beaten by a product that what I've heard is you just spray onto the plate. So technically you're not breaking any rules...
The problem with the document that Mazdamazda posted is there is no specific mention of that and it is therefore open to (mis)interpretation.