Because they abuse their authority. However, this didn't look abusive to me. I don't think he meant to shove her head into the wall. I think he wanted to shove her down for two reasons...
a. She was trying to get away. If you think that her walking is grounds to not forcibly stop her, I'd disagree. She could have picked up the pace at any moment and his best move was to stop her as soon as possible. Also, walking away is the best move. She made it a lot further than she would have because she walked. Had she realized that she was being pursued sooner, she would have ran. Thus she was shoved down.
b. She quickly turned while he was trying to catch her. We have a better view of her body language and face than he did.
Also, she was drunk. She went limp like that because she was drunk, not because he was abusive.
At any rate, the cops were doing what libertarians legitimately ask them to do... prosecute violence. She attacked someone else and thus, that someone else had the right to delegate her just violence to the police.
Really, the only person I think was wrong here is the drunk, violent teen.
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