Equal rights, equal fights.
Exactly. Cops are trained not to look at age, race, or gender. It was one cop, alone, surrounded by the first girl's friends and a bunch of other people. When you have a group that large, when someone gets ballsy and puts hands on a cop, the group mentality shifts and the tendency is for everyone to become more bold and aggressive. When that happens, cops have to rapidly establish dominance before things get out of control.
I used to be a security guard and you pretty much never punch anyone, least of all a woman. There's literally loads of moves he could have used to restrain the woman without throwing a closed fist, in the end it doesn't really matter if she was resisting or if she deserved it, you don't hit girls, especially as a police officer while attempting to make an arrest.
Security guards are in nowhere near the same league as cops. You deal with *******s, sure, but nothing like what cops have to go through. He was actually being quite nice to the first girl, who was being equally *****y. He could have just slammed her ass on the ground (which he probably should have done).
Just TOUCHING a cop is illegal. No exaggeration. These kinds of laws exist for good reasons. Shoving a cop is stepping over a line, one you can't step back from. He did everything right. The chief even said he should have used physical force sooner.