They do it because they can get away with it. Arrest them, fine them heavily (and I do mean heavily!), make them pay for any damages and injuries, and publish their pictures and names in the paper and detail WHY they were arrested. If appropriate, jail them.

For a second offense, double the fine and jail them.

In the US people riot after certain sports victories. And I think the same thing should be done here. I see no excuse for trashing shops, burning cars, peeing anywhere you like, fighting, and other anti-social behaviors. Cops tear-gas demonstrators and I see no reason why thugs should be treated differently.