We have nothing to hide. Our goal is to get arrested anyway. So we tell in advance the police of our actions. The goal is to create a dilemma: either they let us do those actions and everything will happen smoothly (did we say that we are a non violent movement and that we don't destroy anything?), OR they stop us from doing our action and they arrest us which will create more publicity for the movement (why are they arresting those non violent people who get out of their way to defend the living?).