Because the police did all that crap first. At Occupy Oakland, on May Day, the police instigated ALL of the violence. At 3 pm, 5 hours before any window smashing, the police clubbed a girl in the face for no reason whatsoever. And that's just one example. They fight, so people fight back. They shoot tear gas, so people throw tear gas back. They break bones, so people break windows. Self defense.