The argument is that the government can already mandate you to wear clothing to avoid 'public indecency'. They can't tell you what colour or design to wear but they can make you wear a certain amount of clothing to walk the streets. Legally, why not masks?
Businesses can mandate that you wear shoes and a shirt or even a strict dress code (formal) to enter their establishment. Some nightclubs will turn you away for not having nice enough shoes or pants and it's all at the bouncer's discretion.
The TTC can do the same: they already mandate that you 'wear appropriate shoes and clothing while on TTC property'.
How can they enforce it? Rather than leaving it up to the businesses to decide whether to mandate masks in their establishments, the government is mandating the masks everywhere and making the businesses the enforcers. They will not be able to serve you by law if you're not wearing one. Fines will surely be issued if the businesses refuse to comply but the real fear for business owners will be the Karens present in the establishment waiting to be publicly outraged.
We've already seen CP24, CBC, CityNews and others hovering over the parks (in choppers) and 'reporting' on anyone who breaks the recommended measures. Everyone has seen the images of the crowded High Park and the lineup at EB Games for the 'Animal Crossings' release when this all started. The media already stops people on the street and ask them why they're not respecting the 'recommendations' (in an attempt to ambush unsuspecting violators).
Canada can legally enforce the mandatory mask laws with police action but there will be no need. In the end we will enforce it on each other.
I am very interested to hear how other Canadians feel it will play out. Comment below.