I don't think those stickers were ever intended to be put on a car. But the only think I find more appalling than putting a sticker on my car is putting a sticker on my bike!
That being said....I do have a "share the road" sticker - cut into 3 pieces to fit - on my Thule hitch rack on the back of my truck - rather than on the bumper itself. The picture of the bike in a yellow diamond (warning sign) is on the back window, however.
I also got talked into putting one of those God-awful "ribbon" magnets on the back of my truck. But this one is a "Spay and Neuter" one for Friends of Rabbits. Everybody's got their cause.....