You forget that stealing something like that isn't grounds for a lethal shooting, so it isn't a defense. And the cop probably hadn't seen the footage till later, being out on patrol in his car. He only responded to the location of the call and got a description from the dude that got robbed. The altercation with Brown soon after is really only a coincidence that the cop actually ran into the thief without knowing. The fact that he was a thief lends credibility to the story that Brown attacked him. The primary point of defense for the officer is the claim that he was attacked and that Brown was trying to gain control of his gun. The fact of the theft is practically irrelevant except for the fact that it stands as evidence against Brown's character. And the riot, as many riots do, would then be a wrongful reaction to a situation where the wrong things were assumed as a result of the segregation and bigotry that apparently seems rampant in the area. IE, people were so willing to believe that the worst, wrong thing had happened that they acted on emotion rather than straight facts.
@ Erick, that is indeed a black mark on the department, however... it only means so much, especially when the officer in this particular incident was not one of involved in the case you mentioned. It does not mean every officer in the department are so disposed to violence and bigotry. As evidence, this is both highly circumstantial and very weak.