they're our only market in many cases. they can buy things elsewhere, and in fact want to; we don't want them to. we want market access and they're trying to take it away. "cutting off" things us what they want.
if i was in a fight with somebody and i was given the choice between denying them access to goods or services at my own cost or increasing the price of goods and services, it is clear that my self interest would be the latter. i would always choose raising prices over denying access, every time, without serious debate.
perhaps the obvious truth should be considered: if the behaviour of canadian politicians aligns more closely with american interests than canadian interests, as has often been the case since mulroney and the fta, maybe that's what the truth actually is. perhaps this is a charade and the truth is that ford, trudeau, the bulk of the old tory media and the rest of the punditry-political-industrial complex are actually working for american lobbyists and that is why they are talking about "cutting off" trade with the americans at our own demise and cost and their actual longterm benefit, rather than maintaining it by aggressively trying to cash in on it by introducing export taxes.
when some deduction doesn't make sense, perhaps the axioms should be questioned and reanalyzed.