The Police are doing their job. They are trained to listen to orders, so the orders from the top have been to arrest protesters who they think are breaking the law. They feel they are being protected by the force and government so they are more open to stretching interpretation to what is breaking the law. If you hear their press conferences, they believe they are justified in any instances of police brutality. If the police on the ground feel they are being protected, they believe they can get away with more.