You should watch the video, the police are the ones who engaged physically: pushed Pretti, pepper sprayed him, dragged him on to the street, hit him on the head repeatedly, and then got scared and shot. You can protest wherever you like that's legal. If police cannot handle that (start fights and then get scared and shoot) they shouldn't be police. Also, it is absolutely legal to follow police around. It doesn't make sense to make something legal while justifying getting shot for it (unless you want to make it illegal eventually).