Why do people go on the rampage? Really have to insist that the right doesn't get a monopoly on having a pessimistic view of the human condition. People do this sort of thing because they are arseholes and because they can. As Chris says, you need structures where the the costs of rioting outweigh the benefits. The difference between left and right is the former traditionally does not see the costs solely in terms of putative criminal justice. But the left has traditionally seen a role for criminal justice and it's a tradition worth maintaining. Or alternatively you could pretend a bunch of feral scumbags stealing DVD players are at the vanguard of the English revolution - if you want to go on looking like a complete tit, that it.
(And speaking of looking like (or rather sounding like) a complete tit.)
Norm, too, had a (rather more cautiously worded) comment worth reading.