police own a monopoly of violence.
if someone (maybe a corporation, government, neighbour) says you aren’t allowed to sit on a bench for whatever reason, if sit there and stay there long enough, the police will be called.
they will ask you to leave. sure they might ask nicely, but eventually they’ll start to get threatening and become aggressive. and if you still sit there, they’ll eventually beat the shit out of you and force you to move. and they have the right to exert that force on you and you don’t have the right to that force on them.
that is a monopoly of violence. the idea of police brings up the image of brutality. that’s why ACAB. it doesnt matter if there are good ones. the fact that they’re are bad ones to begin with and the system allows them to act freely without worry of real consequences is why ACAB