There is a contingent in this country that believes that the only way to social equality is through violent revolution and the overthrow of the bourgeoisie (they no longer use that term). They foment civil unrest first by promoting the idea that economic success, and therefore social success, is only achieved through oppression and then reinforce that idea by creating situations where that oppression can be graphically catalogued. For example, they will take actions during a peaceful protest which necessitate a police response then photograph that response as evidence of brutal oppression of innocents.
The cops have an incredibly difficult job in a situation like we have in Ferguson. They will see two people "peacefully protesting" by holding a sign but while one of those people is holding the sign to get their point across the other is holding the sign to block the view to their buddy who just took a molotov cocktail out of his backpack and tossed it at the cops.