They do this for one and only one reason. There is no one to stop them. A bully only bullies until he gets his face punched in. These idiots are nothing but bullies.
My thought was during the OWS debacle, several of these wannabe anarchists are separated from the pack, beaten senseless and left with the thought that this will continue until the madness stops.
With all due respect that is completely wrong. Appealing? Yes. Personally satisfying? Yes. But completely totally and fundamentally wrong.
That is precisely the kind of reaction they hope for. It gives them the chance to paint the right as evil and violent. And it gives the totalitarian forces in government (which are barely restrained right now) a justification for action.
"Do not repay evil with evil or insult with insult, but with blessing, because to this you were called so that you may inherit a blessing." 1 Peter 3:9
As we see when the unions protest that they get real violent to the point of killing people in their path. They left gets real violent.