the americans are expressing a willingness, however absurdly, to try to control canadian exports by "economic force". it's like the koalas have taken over the white house and want to conquer the world with the power of the invisible hand. these are frightening comments. however, that's what "51st state" means, right? it means controlling canadian exports.
so, in response to america expressing a desire to control our exports, we scheme up ways to gain leverage against them. then, we get confused when they start talking about controlling our exports some more.
i understand that our oil producers want a better price and are seeking to sell it to a buyer that's willing to pay more of a fair price for it. however, that should answer your question as to why america wants to control it. right?
there's other ways to get a fair price. you could insist on export taxes, as that is where most of the price hike would go anyways. the americans are telling you pretty loudly that they're not happy about you selling that item to those people; they think that item belongs to america.
while being too submissive about something like that is something to avoid, equally undesirable would be being pointlessly argumentative about it.
perhaps you first might try renegotiating the price?