When I worked at a state hospital we had a lot of very dangerous clients (we didn't call them patients because it was a mental institution) and had frequent takedowns because they would often attack each other or the staff, and we had a duty to keep them safe. When we took them down it required several staff because we had to restrain them in such a way that they wouldn't get injured. It's very easy to break a bone while restraining someone, especially someone who is agitated and fighting. You can even kill someone if you put too much pressure on a person's chest, which can stop a person from getting enough air due to chest compression (that's what happened to Eric Garner -- he wasn't choked to death). So with just one person doing the restraint you are more likely to injure the person being restrained and more likely to get injured yourself. Obviously he shouldn't have punched the lady, but I think they actually teach that it is acceptable to hit people in police academies. That's why Rodney King was repeatedly assaulted with a baton, and why Kelly Thomas was beaten to death. We weren't taught to punch and kick clients -- we'd get arrested for client abuse. Police have powers that most people don't have and are trained that way.

And yes, abusive cops can get away with their abuse for years. The culture of silence is real and it works because corrupt cops can make life miserable for honest ones unless the honest ones have a high enough rank that they can make a difference. Richard Cain f Chicago got away with killing people while he was a cop. Fred Pascente was a total scumbag and when he was interviewed before he died thought what he did was funny. But, believe it or not, things are getting better. Years ago the police had something called "the third degree," which was basically beating confessions out of people. It was legal to shoot a fleeing suspect in the back, even for those doing minor crimes. I recall reading about a couple of detectives who held a suspect out of a window of a high-rise building by the ankles. They lost their grip and the guy fell to his death. Sometimes they beat the wrong person and it ends up biting them in the ass. The famous Detective Joseph Petrosino beat the crap out of Ignazio "Lupo the Wolf" Lupo in 1908. Petrosino didn't know that Lupo was a Mafia boss, the head of what later became the Gambino crime family. All the New York Families got together and decided to kill him. When he went to Palermo to do research in 1909, they succeeded.

I suggest reading this new book by Gerry Spence:
http://www.amazon.com/Police-State-Americas-Cops-Murder-ebook/dp/B00TODXQGY/ref=pd_sim_351_1?ie=UTF8&refRID=151ARHE2BD3BT6D15TXW&dpID=51l4NNY1W7L&dpSrc=sims&preST=_AC_UL160_SR105%2C160_