[Assassin]

I dont think a simple police officer can hurt protestors unless they were under direct order, because if he did he could lose his job and might go to jail. So i think its because the higher ups dont want they order to be disturbed and they think if the use brutality and force then the protestors will be scared, stop and go home, if they have orders then its not the officer's fault theres nothing he can do.
But then again there are police brutality due to racism and some people like to join the police  because they enjoy hurting other people and to shoot down criminals ignoring their rights those kind of people are just sick.
The police's job and responsibility is to keep peace and protect the citizens even criminals, and they hurt peaceful protestors then they would have abandoned they job