I'unno, there's an argument that negative attention is better than none whatsoever -- at least the subject is noted and being spoken of, even if the riot itself causes negative sentiment. Opens the way for discussion and attention where there wasn't before. If these riots hadn't occurred, would gorram anyone be talking about the situation in baltimore at the moment? Would the non-violent protests actually garnered any note worth note? We have protests and pushback pretty regularly, from what I understand, and most of the time basically no one outside of the people protesting (and maybe cops or who have you that come in to abuse people a little for the jollies or whyeverthehell they decide to assault peaceful protesters) notice.
Riots are a helluva' lot harder to ignore. You can argue that they're ineffective for opening communication, but that's a question I'd personally say is not clear cut.
That said, as I've seen noted elsewhere, the violent folks involved in these things are usually a pretty minor, well, minority, of protesters. Most people up in arms aren't looting drugstores. There's people coming in to take advantage of what's happening, but that's far from most of the folks there. And there are people that are just pissed off and lashing out and whatever's closest.
As the aside, folks aren't really saying that rioting and looting is a legitimate expression of rage, just an expected one. Tellin' people that this is sorta' what happens isn't saying it's a rightful thing, just trying to convince people that, y'know, maybe we should try to avoid it reaching this point.