This sounds like an ideal situation for the new area denial microwave crowd control device.
I know Vancouver is a fairly liberal city, having visited once for a week and noticed the extremely high incidence of aggressive panhandlers and people casually shooting drugs (I presume heroin) in parks as you walked by, so I bet the police force is accordingly trained in pussified crowd-control techniques, as is the norm for most municipalities frightened of litigation, as they all are.
I thought they let it go on too long. Once things started escalating, they should have marched forward in unison, tased the people egging everyone else on, cuffed them where they lay, stepped over them and kept going to the next one.
They should not allow a SOB to stand 10 feet in front of them throwing things at them.