This is perfectly fine. This teaches the public you don't strike someone just because you feel like it. If you do, you get smacked down. So the NEXT time you are in a situation like that, HOPEFULLY you won't kick a cop and there won't be an incident at all. Easily solved.
ATOT always scream, "common sense." Well the common sense thing to do here is NOT KICK A COP. If there was no attack, there would be no retaliation. I'm not defending someone that fucking can't control themselves. And this would go for ANYONE, not just when a cop is involved. If she did this to a normal guy, she should be punched in the face. It's not a matter of, "oh, he could of stepped back, or he could of just blocked the kick." FU, she should not have attacked someone in the first place! And people that go unchecked are people that will continue with their bad behavior. In this situation, the cop was just putting her in check.