They are just not thinking.

There's a spot where I ride often that I have to cross a bridge. Bikes take the sidewalk here as a. the bike path feeds onto it and b. the bridge would be very dangerous - very high traffic with lots of merging and metal grating deck. Even on the sidewalk its still a place to be careful - its narrow and there are lots of pedestrians and other bikes, there is no railing separating the walk from the cars and there is a large drop of probably a foot or so to the roadway. I had some a$$ teenager (it was a girl none the less) lean out of a car window and scream just to see what my reaction would be. Jeezus if I was a jumpy person - or the woman I was riding with - but she's deaf in one ear and didn't even hear it. If the girls in the car had to stop at the traffic light the little chickee would have found out just how scary a very angry 5 foot cyclist could be.

I highly doubt that she had any idea that she was endangering my life and the person I was riding with. She just thought it would be fun to try to scare the bicycle rider. People like that (and especially kids) don't really think that their actions might have consequences.