I never knew that there was such law - preventing you from dancing at Jefferson's Memorial - it seems stupid as hell, but its a law. I agree with KitKat on this - try to change it by legal means beforehand. Cop's reaction was slightly weird, or maybe that part was cut off from the video. He could mention that law preventing you from dancing there exists. Protesters even asked for a reason, yet he stayed silent.
But i still think that they used excessive force, one guy was standing with his arms up? just wait a while, when other cops finish arresting the rest of protesters, you can easily subdue that guy without using body slams or other such takeover techniques. Both sides are at fault then, cops for using excessive force, protesters for breaking the law and taunting the cops('define dancing' for example).
Dura lex, sed lex.