Look, I am not the lawyer here. But I do know that an indictment is a low bar to meet.
Former New York state Chief Judge Sol Wachtler famously remarked that a prosecutor could persuade a grand jury to “indict a ham sandwich.” The data suggests he was barely exaggerating: According to the Bureau of Justice Statistics,U.S. attorneys prosecuted 162,000 federal cases in 2010, the most recent year for which we have data. Grand juries declined to return an indictment in 11 of them.
http://fivethirtyeight.com/datalab/ferguson-michael-brown-indictment-darren-wilson/Add that to the institutional racism you mention which is clearly a problem in that jurisdiction to put it mildly, and I don't think it's too much for citizens to ask that when an unarmed person is shot, that there is at least a trial. What I've read about the evidence makes it pretty clear that it wasn't obvious. When there's a bunch of conflicting statements and evidence, that's what we have trials for, isn't it?
You're asking people to trust the system. I think there's a point at which you can understand why certain people don't trust that system and I think this is that point.
And yeah, there would be riots if he were found innocent but at least if you let the system play out in full, maybe there would be somewhat fewer people at the riots and apparently that's about all we can hope for in the world nowadays.