What world do you live in? I could just go on a murder spree and when police come to arrest me I can just tell them "you can't tell me what to do", and they'll say "shucks, he's right, guess we'll let him on his way".
By the way, you're being doublespeaky when you say "protest is legal". It sure is. But the vast majority of these altercations come from after a group has been declared a riot, a public safety hazard, and ordered to disperse. They're no longer peaceful protesters when the cops have to use force to shut it down.