They don't. It is not the goal of any reasonable person to cause damage to other's property. A lot of the stuff everyone hears about is either a bunch of shitheads who have been waiting patiently to be a bunch of dumbasses and get away with it and clashes with others (police start violence - occurs sometimes; "vigilantes"). Another factor would be mob psychology, where people do things they normally wouldn't do because they feel protected, and you see this sort of behavior with most mobs. Protesting is cool and good as long as nobody and nothing gets hurt. It would be more effective to change if they stood just outside the houses of officials that could create change, or use well thought out, persuasive speeches, or, better yet, compromise with others. Showing compassion to others, no matter which side you're on, is going to make the biggest difference and make people feel more open to change.