You're totally out of line, and as said earlier, you need to grow up. You're of legal age? Makes no difference. You're not grown up yet. You try to protect people who attack others and you defy authority.
What would YOU have done if you'd been bitten by the kid? I know! CALL A COP!
The police have the authority to "touch" people and to force people to come with them when there's legal reason for doing so. If they're attacked when performing their duties, they have every right to use necessary force to stop the attack and regain control.
Your problem is you don't like authority. You're an anarchist and you want to answer to NOBODY.
Got news for you. That's now how life works. I'll say it one last time- GROW UP.