When do politicians learn not to hold these kind of conferences? If they need to talk, conference calls have been invented and it would save millions (properly billions actually) of public money.
* The conferences are symbolic. They send the signal that the various leaders are in some kind of accord. Politicians could and sometimes do hold the conferences in places where mass demonstrations are impossible - like Dubai. They are holding the conference in Toronto as a symbol that they can hold it in Toronto. [edit: wrong city...]
* The conferences provide funding, a pretext and a fairly safe venue for law enforcement to test it's latest tactics and overall readiness for more serious unrest. Sure it's "an inefficient way to spend tax payers money" but the leaders only talk about that when their friend's pet projects aren't involved (note how US Federal spending is heading towards 50% of GDP without the government providing basic welfare state type services).