How were the police justified in this case?
Because I don't know the details of the incident involving the man who died, it would be foolish of me to claim this or that action the police took was justified, or not. That's what grand juries are assembled to do.
But this kind of rioting has sometimes happened where there was no police misconduct at all. What touched off the Watts riot of 1965, for example, was a white highway patrol motorcycle officer pulling over a young black man, Marquette Frye, early one summer evening, for driving in a way that made the officer suspect he was drunk. He had every right to do that. Police are obligated to stop drunk driving whenever they have good reason to suspect it, because it is a serious threat to public safety.
The officer's suspicion apparently was justified, because Frye failed a field sobriety test. He then became belligerent and began to walk away, cursing and shouting that police would have to kill him to stop him. By then, about fifteen minutes after the stop, other police had arrived, as well a crowd of about 250 sullen locals. These included Frye's mother, who lived nearby--when she first arrived, she had scolded him for drinking--and his brother, who had been a passenger in the car. When officers tried to arrest Frye, who had walked into the crowd, some of these people, including the mother and brother, interfered with them. Police then arrested all three of the Fryes, which made the hundreds of interlopers furious.
The brouhaha at the scene of the car stop spread quickly throughout that area of Los Angeles, becoming more and more violent. The disorderly behavior and disrespect for the law the people in this crowd had chosen to show gave many other people in that area of Los Angeles, almost all of them black, a pretext for engaging in several days of rioting. Thirty-four people were killed as a result of the many felony crimes committed. These crimes also cost about $40 million in property damage, equivalent in today's money to about a quarter of a billion. I suppose that out of racial sensitivity, the officer should just have let Frye keep driving, and if he had run down a couple little kids a few minutes later, so what?