The grade argument is debatable. Yes, if you want a bonafide, completely authentic as in France croissant it is less easily obtained (sidenote -- there are makers who
do make the higher fat, lower water butter, but there is very little demand), but if a maker is choosing between a imperceptibly different Canadian standard butter and margarine (the former indistinguishable, the latter a vile abomination), and you think that minuscule difference is the reason, you are misled.
Further it isn't expensive. Large quantity butters purchases, including special higher fat versions that aren't generally on grocery stores, are an insubstantial percentage of the cost of making a croissant.
https://vancouversun.com/news/local-news/canadian-croissant-...
But again, the reason croissants are generally horrendous in Canada is because people just don't care. If people don't care you end up with the weird bready-things that are marketed as croissants. This is a bit of a personal peeve having just gotten back from Belgium/France a couple of months ago, and being spoiled by every hotel having extremely good croissants.
The debate over supply management is revisited at every election, and overwhelmingly Canadians buy into the notion that supply management has benefits in certain industries.
As an aside -- it's cheaper to buy milk and butter in Toronto, Montreal, Vancouver, and virtually every other urban center in Canada than in San Francisco, NYC, LA, or any other large US center. US dairy dumps their excess on secondary markets like Detroit, and it's always the "Windsor versus Detroit" sort of comparison that we hear parroted.