That’s not empirically true. I pay for water as a flat rate in Quebec as part of my municipal taxes, as do all of my neighbors, and I don’t see people engaging in flagrant water wastage. Lawns routinely go yellow during the hottest parts of the summer, I rarely see people washing their cars, and low flush toilets are getting increasingly common.
Ok. The EPA estimates that the average American uses 82 gallons a day as of 2015, which comes out to 310L.
EPA link
By contrast, McGill University cites that the average Canadian uses around 329L a day.
McGill water usage page
Montreal, specifically as an unmetered water city, estimates 327L a day.
City of Montreal annual water usage report
I’ll grant you that Montreal does seem to have slightly higher usage per capita. But I’m not sure the extra pain in the ass of managing water meter infrastructure would be meaningful to reduce water usage to be in line with metered locations, when we’re talking about a difference of 17L a day.