I can understand where they are coming from. Teen aged boy is shot dead. They don't believe Zimmerman's story of self defense, even though the jury did believe it. They just want revenge. That's natural. They used to declare blood feuds over less.
The purpose of courts and the justice system is to prevent just that. The blood feud was still alive as recently as the Hatfields and the McCoys. Responsible public officials are supposed to talk about the progress that has been made and to discourage violence.
The black community has things that anger them that go way back, back before the Civil War. Things like slavery, poverty, lack of respect, rotten schools, and others. A lot of these issues trotted out and attached themselves to Trayvon Martin. Yesterday's demonstrations are partly about Zimmerman's acquittal and largely about those other issues.
The civil rights movement of fifty years ago addressed the legal and political side of this. The movement had concrete policies, desegregation and voter registration that had concrete remedies in laws. Those laws got passed, and are still in place. This ground was well covered by the end of the sixties. Today's black leadership cannot think of anything else that could be addressed by law. So instead they talk about "having a discussion on race", which is a code phrase meaning, "We don't have any ideas to fix things, but we are still unhappy."