They used to go on the rear license plates, but some people figured out how to peal them off intact and steal them so they could put them on their own cars. Even if caught they only could be charged with a misdemeanor. The sticker that goes on the inside of the windshield has the license plate number printed on it. Not only would it be suspicious to have a sticker that doesn't match the license plate for most cars it would be required to break into the car to steal the sticker. That's breaking and entering which is a felony.
I think the law changed breaking and entering vehicle to misdemeanor.
Interesting info. I did not know that. It helps make sense then.