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.
History written in textbooks tends to teach the narrative that was sponsored. Modern pop-history tends to teach what will sell the most copies, so history is teaching us all kinds of selective facts about life.
Regarding force - force needs to be applied for anything to move - that's just a fundamental law of physics :) The question is where and how do we apply force and in what quantities?
My stance is that witch hunting did not solve the issue of witches causing droughts, scientists along with engineers and people willing to work today, for a better tomorrow, solved hunger in many places of the world.
Now imagine somebody that does believe that his or her witch hunt of year 1543 did fix drought for a decade in his/her town. What evidence could you possibly provide to convince them otherwise? They know there was no drought for ten years, they know they burned a witch in 1543. Now if there is a drought ten years later, clearly we just didn't burn enough witches!
This is the difficulty in dealing with people who rely on emotion, not cold hearted rationality, to solve problems. Cold hearted rationality has its dangers too and one can argue until the cows come home that we may have been better off never industrializing, never inventing agriculture and sticking with hunter gathering, at one with nature and all. Not me, I like hot showers, anti-biotics and all the other modern amenities too much. I'm in the rationality over emotion camp. The people out there protesting I'm guessing are in the other camp and that's totally fine with me.