Right, but they will not have that fear in Canada because you pass through CBP before getting on the plane. Their fear is getting fined by the country they transport a passenger to and having to return that passenger at their own expense. Because Customs in done in Canada before getting on the plane, the traditional fear does not exist because the passenger has already been cleared into the United States; the airline cannot be forced to return the passenger or pay a fine. It is an unusual exception that applies to Canada-- the airline has nothing to lose in that regard. If it were not Canada, then I'd agree with your position.

If the flight were overbooked, I can picture the airline making an issue, though.