If a cop identifies himself and someone walks the eff away, they will pay for that decision in court. Keyword: walking. Does the cop have the right to take her down and arrest her? Of course. Does he have the right to run up to a subject that is walking and throw her into a wall for no rough ridin' reason resulting in her falling to the ground? No. He could have run up to her grabbed her arm, pinned her against the wall, and cuffed her the eff up.
His job is the job he chose. If he can't make decisions on the amount of force needed in situations, he doesn't need to have his job. Cry me a river about the stresses of a life that someone chose. Your pity part for his chosen job is no different than my pity party for the way she was treated. They both made life choices that took them there to that spot, one's life choice resulted in assaulting a drunk teenage girl.