When is Violence Necessary?

Starting last night, there has been rioting in Tottenham and Enfield (depending on who you depend on for news). These places are in London, which isn't in America, a fact that I found only mildly disturbing. I'm not posting any links about this because the details are still sketchy at best all around, but also because my sources (believe it or not, I have friends that tell me things) tell me that the news might be skewing things toward the "not facts" category.

What I do know is this, a country not known for rioting and violence has experienced rioting and violence. Because I've been getting a steady trickle of facts, I've been able to ask, what would I riot for? And I think I know what would lead me to violence.