I am not justifying vandalism, but you do realize that a Darwin Fish is obnoxious and offensive, don't you?
Do you know where the fish symbol came from, and why it was originally used?
There is such a thing as "fighting words". Clueless idiots who put a Darwin Fish on the back of their automobiles should realize that, and not be entirely suprised when people take offense at things that are offensive.
The same clueless idiots you refer to would be just as likely to take a crowbar to a car with a BUSH/CHENEY sticker, or an SUV without any other ornament.