They had a lot of police around, high up as well filming so imagine their tactic was to sit back, get evidence and make arrests after.
That's fine - and I understand that as an operational tactic. But if your 10-year old lad gets hit on the head by a Becks bottle some coked up nob has launched into the air just for the lolz, then on balance I'd rather they stopped filming and went and practiced a bit of crowd control. Can't they get civilians to film / gather evidence?