I completely disagree that leaders of a protest movement should say that violence is an unacceptable tool in their movement. For one, there is no consensus around what constitutes "violence" -- some people consider shouting to be a form of violence, others consider property destruction, and others restrict their definition to bodily harm. Then there's the question of whether self-defense justifies violence, and the related question of what constitutes self-defense -- if you believe you're going to be forced from your home, is it self-defense to resist?
There is a real history of "divide and conquer" being used by the state to publicize and in some cases invent real and imagined differences between groups. The memory of COINTELPRO -- specifically bad-jacketing and anonymous letters -- is very strong in today's Left (and understandably so, given the success of the tactics at neutralizing groups like the Black Panther Party and the American Indian Movement) and trying to get half of a protest to condemn the other half should rightfully be met with suspicion.
I don't think people need to answer for things they did not engage in or intentionally facilitate. Also consider: agent provocateurs have been used in the 20th century (a matter of fact) and the 21st century (perhaps merely a matter of suspicion) -- http://en.wikipedia.org/wiki/Agent_provocateur#United_States and combining the tactic with your concept that everyone who participates needs to answer for everyone else who participates is results in some pretty unfair bullshit really quickly. Especially for any event that's publicized in any way, you really can't control who shows up and what they do, and it's unreasonable to expect people to.
As for the actual incident, based on one of the first-person accounts it sounds like the window was broken from inside the bus. If that's the case, that actually shows that the participant greatly increased the risk to themselves in order to minimize the chance of bodily harm. If you don't draw a distinction between actions against people and actions against property (a moral position that is strange to me - surely there's a difference between smashing in windows and smashing in heads!) perhaps that doesn't matter to you, but not everyone sees the same way, nor do people need to.
If you want to talk about violence, focusing on what you've defined to be the violence of the protesters is also greatly limiting the discussion. Evictions are characterized by armed sheriffs showing up to someone's home and forcing them out. I think evictions -- which happen systemically and all the time -- are much more violent than an occasional window being broken, and I don't think I'm alone in that belief. MLK also used armed guards to protect himself and while it's easy to find quotes of him calling the US violent, I challenge you to find a quote where he condemns members of the movement who believe in violence. He viewed non-violence as strategically superior, yes, but his cooperation and coordination with groups that support self defense only increased over the years, up until his murder.