We are now aware that the event in Ferguson, MO, was similar to the Trayvon Martin case in this respect: an unarmed black male was killed after attacking an armed person.
Before the facts in the case were known the locals were protesting the common myth that white police shoot innocent black people every day, if not every hour, and get away with it. Later, some of the crowds engaged in looting and arson. At one point the law officers were told to let the crowd engage in illegal behavior. As crowds are allowed to get away with criminal behavior they will tend to push the limit of what they can get away with, just as children will. In response to that, the police must be ready to protect themselves from unfettered behavior and so they arm themselves heavily.
If the police could depend on the crowd to obey directives the police would not have to wear heavier protection.
Andrew Woodward, Tucson