people generally protest because they enjoy it, and for many it raises the adrenaline, often leading to violence or civil disobedience. use of cannons and plastic/rubber bullets inflames these reactions and is often counter productive.
a better way to control such riots/protests is to hem them into a confined space and then just wait until they get bored or fed up and go home. it also saves the public purse from all the ipcc investigations, inquiries and claims.