It seems rather common sense to me. Most people riot because of the cause but because they feel protected by the anonymity they gain being in a large crowd and they feel they can loot and destroy with little chance of being caught. As time goes on the crowds get bigger and the sense of safety grows. Want to stop a riot? Remove the feeling of safety from the equation and quickly. You can't arrest everyone but you sure as hell can flood the crowds with gas, rubber bullets, and bean bags. The idea of running around all hooligan like will quickly look much less appealing. EDIT: Oopsie on spelling.