As long as we have a justice system with less teeth than a denture user and a prison system with more amenities than a bleedin' hotel scenes like this are hardly surprising. Add to that the underclass that has been created over the last few years and something like this was bound to happen somewhere in the country.
As for why water cannons weren't used Mr G, 3 little words would have shot that idea down before they even got out of the garage. The abortion that is the Human Rights Act