I believe such signs should be illegal. They block your view through the rear window needlessly which can result in dangerous situations. If you wish to advertise on your car place signs on the doors.
@Edward, It would do me no good to have a sign in my back window as I always have a trailer behind my truck. My rear view mirror and window is useless to me. I have learned to use my side mirrors instead. The older I get the more I appreciate not having to turn all the way around to back up too.