Olive,
I just checked in Wikipedia,.
Actually the cops in Rodney King's case were in fact prosecuted:

Federal trial of officers
After the riots, the United States Department of Justice reinstated investigation and obtained an indictment of violations of federal civil rights against the four officers. The federal trial focused more on the evidence as to the training of officers instead of just relying on the videotape of the incident. On March 9 of the 1993 trial, King took the witness stand and described to the jury the events as he remembered them.[20] The jury found Officer Laurence Powell and Sergeant Stacey Koon guilty, who were subsequently sentenced to 30 months in prison, while Timothy Wind and Theodore Briseno were acquitted of all charges.

Cultural impact of the event
The video of the beating is an example of inverse surveillance of citizens watching police. Several copwatch organizations were subsequently organized nationally to safeguard against police abuse, including an umbrella group, October 22 Coalition to Stop Police Brutality.[21] The clip to Ministry's song "N.W.O." features a re-enactment of the assault video, substituting a woman dressed as the Statue of Liberty for King.

After the riots
King was awarded $3.8 million in a civil case and used some of the proceeds to start a hip hop music label, Straight Alta-Pazz Recording Company.[22]

see more here: http://en.wikipedia.org/wiki/Rodney_King