I guess I'll be the one person here who's against it. Sure the girl had it coming, but you just don't punch a girl in the face. I would rather the cop had tased or maced her.
Easy to say in a one-to-one situation but he was being ganged up upon and hac to deal with that ASAP. There was no guarantee others weren't soon to follow.
A quick punch in the face gets the point across very quickly and effectively.
Besides, tasing is not always a solution without ramificatiions, as recent news stories will attest. ...and mace isn't guaranteed effective in stopping flailing fists.
Ultimately, you fluck with a cop, particularly physically, doing his job, be prepared to take what you get.