Students trying to enter with unexpired visas but expired student status account for a huge chunk of denials at ports of entry. Maybe not the land crossing from Canada so much, just in general. Refusals for permanent ineligibilities are quite rare, though possibly more common for Canadians than other nationalities, since almost all others have to have either a visa or an ESTA review before showing up at the border in the first place, and there are thus fewer surprises for those travelers.

Again, the "lifetime ban" drama is undermined by the fairly liberal grant of waivers of the lifetime ban. An otherwise good traveler who has quit using is almost certain to get the waiver. Canadians, having visa-free travel, live a more complicated and expensive life than most in this regard, since anybody else can just apply for a tourist visa and have the waiver bundled into the application (hence the few weeks figure) while Canadians have to deal with a whole expensive rigmarole--but the results are usually the same.

Bottom line, there are oodles of people--some of them even former drug traffickers, never mind drug users--who are able to travel to the U.S., with some difficulty, true, but being turned around with scary talk of permanent bans is not the same as actually being permanently banned.