There is a point where police who are in attendance at those simply want to let the protest occur, to let it continue, as long as it's peaceful—the operative word being “peaceful”. If there are hundreds or perhaps thousands of people who are dressed in some manner that would express their views but who remain peaceful, then police officers are more than satisfied to let it just simply occur and everybody goes home at the end of the day.
The intent of those individuals is the most difficult part to determine. The intent of somebody who shows up with weapons in a bag, and a mask and gas mask, is not, in my view, to protest or make a statement: it is simply to do damage.
That's the difficult part to determine, who is doing which.