As is normal in the abnormal situation of rioting, mainstream press and other public commentary has sought to define the motives for, and thereby define the meaning of, violence. From what I've seen in this case, explanations rest on violence being "protest" carried out by "youth". But protest of what exactly? Protest against the killing of a person who presents absolutely no threat? Protest against police violence? Protest against a sort of terrorism against citizens in the public realm? I'm an outsider working on sketchy information, but it seems to me a couple things are relevant in pondering this question. First, there wasn't a riot on the BART platform when Johannes Mehserle ended the life of Oscar Grant. There were people around and there was an immediate collective sense that a horrific act had just been committed. Moreover, some bystanders overtly empathized with the murdered man in a "that could have been me" fashion. Second, the marches that followed in the days after the shooting were framed by the marchers (I think) at least in part as a protest against how the OPD was handling this case. Mahserle wasn't brought in and charged until the 13th and at that point, violence in the streets had largely ceased (except for some police actions which might be called counter-rioting). It seems plausible to think that violence on the part of the public was focused on this non-action on the part of the police. OPD, Riders and all, have a bad reputation among the public in Oakland. There isn't a lot of trust that they will act in a just manner, particularly when it comes to holding a fellow police responsible for a crime.
The other element in explanations of the violence has to do with the age of rioters. The rioters are described as youth, but the arrests seem to be mainly of people in their mid and late 20s. Mehserle is 27 and yet doesn't get labeled a youth police (though his lack of training is often noted). What's the "youth" label meant to do? Portray violence as normative among whatever population is being described? Excuse the violence as an ill-considered response by those too young to be more reasoned in response? Whatever the answer to that question might be, I am certain that the overall effect is to undercut our ability to treat this violence as being a serious, meaningful act that should be contemplated.