Maybe all of the following is obvious and widely-known; I haven't seen it discussed, though.
The "distinct society" portion of the tuition conversation has been mostly about the transfer of authority over higher education from the Catholic Church to the Quebec state in the wake of the Quiet Revolution, and the conscious commitment to work toward a social-democratic model of tuition-free higher education. But it seems to me that there's also a strong relationship with language-population politics.
The Quebec higher education system has several relevant distinct features:
1. CEGEP/ college education going to grade 13
2. Following directly from that, a 3-year university BA
3. A differentiation between tuition for in-province and out-of-province Canadian students-- standard in the US but, I believe, unique in Canada Update: not unique, I'm told in comments, but I'm having trouble coming up with general information. So far it looks to me as if Ontario, BC, Alberta, and Calgary all have uniform Canadian tuition rates, without provincial differentiation. More information, please!)
4. Very, very low in-province tuition-- not 0, but much closer to 0 than to tuition in Ontario or California.
5. Unusually high provincial levels of taxation
I treat (5) as part of higher education policy because defenders of low tuition insist that students aren't trying to avoid paying for their educations; they'll just pay for them later through taxes rather than up-front through tuition; and that this moreover prevents low tuition from being a regressive subsidy to the middle- and upper-class students who are most likely to attend university. And of course it's importantly connected to (4).
Now, the first thought I had in looking at all of this was, "anomalously low tuition and anomalously high taxes to pay for it can go together in a closed society where the same people spend their whole life cycle in the same tax-and-spend system, and the closed society is a convenient assumption for some social democratic modeling, but its empirical falseness means that the micro-level fairness story fails. You'll get people getting their cheap educations and then leaving, while others who have paid full price for a university education elsewhere, or even out-of-province tuition here, migrate here and then pay again through the tax system." Now, one unattractive feature about that from my perspective is that it creates a possible sense that people are doing something wrong, shirking their fair share of the burden for their own education, by out-migrating; I think that's an illiberal norm to run a society on. But on its face it also looks fiscally unsustainable: everyone's incentive is to get the education and then get out. And then I thought to myself, "discouraging out-migration is an important part of the preservation of The french Fact. So I'm missing something."
Separate the population into three groups, and look at how the system works for each.
1) Out-of-province students have roughly neutral incentives to come to university here, but a disincentive to come to university and stay. Out-of-province tuition is roughly comparable to tuition elsewhere in the country (though still lower than Ontario), and out-of-province students get the standard 4-year degree since they didn't go to CEGEP, so if they just come get a BA and leave again they're neither getting any special discount nor paying any special price. But if they come and stay, then they've paid 4 years of normal tuition rather than 3 years of cheap tuition, and then they spend the rest of their lives paying taxes as if they had benefitted from the discount rate. (The same is true-but-moreso for international students.)
2. In-province anglophones have an incentive to do what I described above: get a BA on the cheap by paying three years of low tuition, then migrate out to anywhere else in North America where their taxes will be lower. The incentive to stay local for the BA is very steep.
3. In-province francophones face the same financial incentives as in-province anglophones: a huge incentive to stay local for the BA, since the three-year low tuition degree is vastly cheaper than a four-year normal-tuition degree elsewhere. Then-- here's the part that puzzled me-- they have an incentive at the margin to leave when the high taxes kick in.
But exit in post-collegiate early adulthood is a lot easier for anglophones. They've got, roughly, the whole Canadian and American college-educated labor market open to them, and they enter it on an equal footing with those whose educations were anywhere else in North America.
If English is neither your first language nor the language of your university education, it's a lot harder to suddenly jump into the educated-labor market of anglophone North America at age 22 or 25. You're starting at a disadvantage in that market that doesn't apply if you stay close to home. If, by contrast, you had left home for an English-language four-year education, you'd be a lot more likely to, as it were, defect, and take advantage of the economic opportunities open to anglophone university graduates in other parts of the continent.
So the system as a whole acts as a financial disincentive to permanent in-migration from the rest of North America (and NB that French citizens pay in-province tuition rates, not international tuition rates) and as a marginal incentive to out-migration for anglophones once they've gotten their college educations. But for francophones from Quebec, it acts as a strong incentive to stay at home for university education, a moment when there might otherwise be an especially high risk of permanent out-migration, and a marginal reduction in their ability to out-migrate later.
In other words, even if some number of high-earning francophones leave (and therefore never "pay back" the cheap university educations they receive) the system broadly tends toward making francophone Quebec a more self-contained economic world in which people do spend their whole life cycles, while simultaneously subtly encouraging anglophone out-migration and discouraging anglophone in-migration.
This, perhaps oddly, makes me slightly more sympathetic to the system than I would otherwise be. (It also, of course, makes it more sustainable than it would otherwise be; it significantly retards the get-your-cheap-degree-then-get-out dynamic.) Francophone Quebec does need to be a partly self-contained economic world to be sustainable; a large steady outflow of 18-year olds who never came back could be the beginning of a downward spiral in the viability of the French Fact. (Note, too that a bloated civil service is often a part of this kind of system in postcolonial societies; it provides jobs for a surplus of locally-highly-educated workers.) Of all the possible policies to sustain the French Fact on a population basis, this tax-and-subsidize policy is on the low-coercion side. (It might, probably does, depress the overall prosperity of Quebec, and that has to go into the calculations too; in the long term, la survivance will depend on an economy that is successful, competitive, and attractive, not just one that is self-contained enough to discourage emigration.)
But-- if I'm right about all this-- I do think it's worth acknowledging the uncomfortable truths that the system operates to diminish the mobility of local francophones, indeed depends on doing so, while simultaneously greasing the slide out of town for local anglophones.
This is all back-of-the-envelope modeling, and I'm entirely open to correction and instruction in the comments.
See also: Kymlicka and Patten, eds., Language Rights and Political Theory; and an article of mine defending the compatibility of ethnocultural federalism geared with an emphasis on preserving the national minority's culture with liberalism.