hmm.. Theres a big difference between what these protestors did and what happens in a riot though. Riots don't tend to be premeditated events, they're a sudden violent outburst.
Edit : Also, from the riots I've found myself in, I don't think the issues themselves were actually marginalised by the violence. In fact in all cases it has been a given that if it wasn't for the protest kicking off the issue wouldn't have attracted the media attention it did.
Saying that, I'm talking of spontaneous violence, usually caused by heavy handed policing. I have no time for the actions of these animal rights protestors.