It also takes just a few rotten apples on the side of the police to make it go violent.
Protesting is largely a modern witch hunt. It lets people blow off some steam, rather ineffectively in the modern protest case, because you don't get to burn anyone alive at the end of it. Enough people long for that witch hunt finale, and that's why protests turn violent so often, it's the modern day equivalent of burning a witch.
I don't know if protestors realize that it is up to them to come up with ways of fixing systemic issues they're upset about (people in power are fine with the way things are, by definition) - burning down your local neighbourhood or yelling out in the streets can only lead to the powers that be going 'ok fine, go find some witches to burn to appease this mob'. It's never going to fix actual systemic issues, but perhaps burning witches is good enough, given our history, and going in circles indefinitely is the way of this species.
Counter point - the Civil Rights Act of 1968. People pay attention when they are forced to. I won't pass a moral judgement as it's not my place, but nothing will change until force is applied. I think that is the lesson history teaches.