Because it gives those without a voice or influence some sort of pressure-relief.
It does a damn good job of making those who believe they do not have a voice or influence who use this tactic look bad. Consider this article posted a while ago. Now put yourself in the assaultee's shoes. Are you going to think "Maaaan I was totally being part of The System, I feel bad" or "Why did that dumb fucker decide to steal and smash my shit when I was walking down the street minding my own business?"
I'll feel bad for The People when The People stop being fuckwads who seem to believe that their disadvantage is an excuse to be uncivilized. Their actions (and some of the positions a good bit of them hold) alienate people who might otherwise be their allies.