Unfortunately we still live in a society of laws and one of those laws is that you're innocent until proven guilty.
So, if the crown is unwilling to lay charges on some rioters because of a lack of evidence, then we have to suck it up and live with it.
So, to avoid this problem. Next time Vancouver decides to have a riot may I be so bold as to suggest a large military presence, a banner reading, "disperse or you will be fired upon" a roll of white cotton tape to set the proverbial line in the sand and alot of live ammunition.
It worked for the Brits in Malay and trust me it'll work for the retarded rioters in Vancouver.