They are sadistic, cowardly a-holes. They know it's not o.k. but they must get some cheap thrill out of hurting or threatening someone who is helpless to do anything about it.
An idiot was arrested in L.A. some time ago when his friends videotaped him shooting paintballs at pedestrians, cyclists, and people waiting for busses. I think the cops saw him do one and pulled him over and saw the tape.
They showed the tape on the news and a cyclist he shot went down and the fools laughed.