Foreign operators are able to offer things - like full roaming and data in Canada - which are outside the direct controls of the Big Three and their bribed CRTC minions.
I have a friend who pays (I think) about $20/month to get full coverage in Canada, USA, most of North America, South America, Asia - unlimited talk and text, along with 100GB LTE data (with indefinite rollover of unused data every month) - from a Singapore-based operator. She also gets tons of gifts, prizes, rewards, savings, and promo stuff from them - they value long-term customer loyalty. (Hard to get exact details from her, since she just laughed when I described Public Mobile's offerings, I think she just doesn't want to brag.)
The catch? To subscribe to this company you've got to be a citizen of Singapore with an address in Singapore. But take a quick look at their front page plans to get an idea of what's out there. $10/month for 5GB unlimited local talk plan, $20/month for 100GB LTE unlimited roaming plan, $30/month for 250GB LTE unlimited roaming plan, etc.
https://www.circles.life/sg/
The rules about who can (and who cannot) be a subscriber are imposed by the foreign operator itself.
They're not rules imposed by (or enforceable by) the CRTC market protections wireless regulations.
Other foreign operators have their own rules. Canadians in Canada will be able to subscribe to better and cheaper wireless plans in Canada (and USA/etc) through foreign providers than through Canadian providers, options exist now, more and better options become available every year.
