Based on my experience living in LA during those years, Jeanna, I think the speculation (which I've heard as well) is one of those too-simple sound bites. Many inner-city blacks (and Latinos) in the early 90s had come to believe the justice system (police and courts) was stacked against African Americans. The ensuing Rampart scandal showed this view to be largely correct.
Thanks to the magic of television and a video of (mostly) white men whaling away on an unarmed black man, the Rodney King incident came to symbolize that view.
In that context, it shouldn't surprise us that an inner-city jury was extremely, even overly, skeptical of evidence that came mostly from the police and police labs. (The jurors were wrong, of course, but there were reasons why they viewed the evidence as they did.) Not exactly payback for King, but certainly out of a related context (
I can't speak for the motivations of black activists in the Imus scandal, but to me, they seem to be doing what they always do (and did before the Duke case). But we should keep in mind that the firing of Don Imus was effected by mostly white executives at mostly white corporations.