Keep in mind that the oil is being produced and used regardless. For Americans, it's a matter of whether the oil gets shipped to Texas by a new pipeline, or by far more dangerous rail. (Yes, you argued against this idea a year or so ago, but that's exactly what has happened.)
Granted, there wouldn't be quite as much oil shipped south, because being dependent on rail makes another pipeline to the left coast to serve China look more attractive.
For Canadians it's also a national security issue. If America gets too dependent on Canadian oil it becomes a national security issue for the US. And THAT makes it a national security issue for Canada; we've seen what the US does in the name of national security.
And so we've brought in a lot of investment in our oil fields from the EU, China and elsewhere. The new free trade agreement with the EU, signed but not yet ratified, has side deals in effect for EU oil workers to work in Canada. There's a big push for another pipeline from Alberta to the left coast to serve China, and to refineries in eastern Canada.
Sending the oil to China and elsewhere would also cut back on the roughly $1,200 per Canadian per year that we subsidize U.S. energy consumers.