I've explained this. He was trying to cuff her. "Having her on the ground" was not the goal, you see. This was merely a step. The next step was to get her on her stomach and cuff her hands behind her back, but he couldn't do it because ... she was resisting! ... can you dig? can you grok? can you expand your mind into realms beyond your own political agenda ... Oh! I didn't think so!
You have your own little reality ( shared by millions ) where "having her on the ground" amounts to some kind of moral neutrality with which he should have been satisfied, kind of like our mideast policy, I guess. Well, the CHP officer had not been informed of this. Whose fault is that? He proceeded agressively, by the book, by the doctrine, towards the goal of cuffing the "suspect", if that's the generic term. And as I've stated, he achieved this goal within seconds.
So now, what? Millions, tens or hundreds of millions, in lawsuits? Riots? Dozens dead? OK. More fun news.
Oh yes, that justifies the punching!
No, it doesn’t.
She was on the ground, you compelling someone to roll over.
And that is not done by punching them into submission.
Why do you think cops grab your thumb to pull your hand behind you to cuff you?