Ask yourself, as well, why did the cops beat Rodney King? He was complying with them. Rodney King was a stranger to those cops. Think about both sides of your question.
You may as well ask why did the French peasants attack the Bastille and the guards there, when most of those peasants had never seen the inside of that prison? And the innocent folks in the streets of Paris during that time? Or Kristallnacht? People rioting in the streets and beating the innocent there, too! This is where context comes in. It’s not like every day, the unwashed masses are mobbing around beating random innocent people. Something has to set that sort of behaviour off. Think about the beliefs people carry around that get sparked like a fire and sets people off.
You have to accept that there will be times where many people will choose violence in the heat of the moment. And if you want to fully understand it, you will have to stop being shocked or outraged every time something like that happens, and pay attention to why it’s happening. Because violence happens. Everywhere. Committed by all sorts of people. And it will pretty much happen again and again, as long as human beings are animals with emotions and passions, whether you like it or not. I don’t like it either. I thought the riots were horrible. Mr. Denny happened to be at the wrong place at the wrong time. How much would you like to bet those people would rather have beat the crap out of one of those acquitted cops instead?