I also believe neither side has a right to riot.
Agreed.
Going into "not educated enough to speak on this topic" area here, but whatever. I think there's a real slippery slope that happens when this is the scenario:
1. People assemble to object to "thing"
2. It starts peacefully (I believe the literary folk call this line "foreshadowing")
3. Local government decides that the number of people is above their comfort level
4. Local government decides to say "fuck the First Amendment" due to whatever reason it likes
5. I won't write out the rest other than to say "hilarity ensues"
On a sort of related note, I watched The Trial of the Chicago 7 recently and I found it to be a pretty good movie. This movie also sheds some light on what @UtahJoe was saying - shit was way way way worse back then. The reality of the cell phone renders that scenario impossible these days. There's no way law enforcement would go hog wild on protestors with that many cell phones around.
Also of note - the movie Fruitvale Station, which tells a story where the incident was caught on film. Of course, the white officer that shot the black guy ended up serving a year after being sentenced. If you really want to read yet another story about a fucked up system, it can be found here. So I guess these guys can't totally get away with it. But it's not exactly eye-for-an-eye territory there.
I digress.