The common argument is that unless you go to a few top tier universities like UToronto or UBC, most canadian universities aren't that great, cost a LOT (compared to european universities), and pay in Canada doesn't justify paying that much for going to school there. Note that this isn't a situation where you're going to grad school because you want to go to grad school. This is specifically a way to game the shitty system because going to school in Canada is a guaranteed PR after, which is why it's called "buying a PR". I don't believe canadian education is worth the price tag, but the question is whether or not the PR is...
As to the first point, the 'greatness' of a university depends on your course of study, surely you'd know that. I myself have a stem background and I'd take a McGill or Guelph over UBC, UofT and most universities in Europe (outside of about 5 universities in Germany and the Netherlands and the top tier UK ones).
The fee argument I can understand, but then again the average fee to study in Canada is less than the UK and Australia, even for similarly ranked schools. If saving on tuition and board is the concern, they are better served pursing opportunities in Europe, but might need to learn a new language, less welcoming society, etc. I think people take the tradeoff and settle for Canada regardless as long as they can afford it.
The last one is interesting, like it or not, most students from the developing world make the international education journey with the long-term view of migrating. Outside of a very small percentage that do it because of a genuine reason to advance academically and return back home immediately, staying back features heavily on the minds of students allover the world. These countries know it too, hence the often 'enticing offer' is staying back after school to find work. Nothing about gaming the system there.
When I decided on grad school, asides from the possibility of funding and working on something that I was passionate about, the path to PR was a very important consideration knowing where I was coming from.
