I think they are doing that for their kids who get those stickers from school. It's a family thing.
I can totally understand it, and I am totally for it.
The dumb ones, to me, are the ones that read, "My kid beat up your honor student." Why would they want THAT on their bumper? It's mean. Also, the ones that read, "My pomeranian is smarter than your honor student," are equally dumb.
It's the battle of "on the road reading materials." I appreciate drivers want to give us something to read at a red light, but let's not have rivalries.
The real problem that I can see is for the child. Suppose just one marking period they get even a B-. Off goes the sticker or the kid has to look at it for four months until the next report card. They have pressure to live up to a bumper sticker.