Wow, let me put it this way, you are absolutely wrong. In fact, you couldn't be more wrong.
NAFTA requires no government intervention in the normal function of the energy market. It also requires all consumers of energy in North America to be treated equally. Nowhere does it set or require quotas. See reference here: http://www.parl.gc.ca/content/LOP/ResearchPublications/prb0633-e.htm
Furthermore, in this particular case, the US obviously does not want Canadian oil. When the Northern Gateway is completed, feel free to buy as much as you want. I'm sure the producers will be glad to sell to anybody that can get a tanker up to Kitimat.
Canada is not really interested in "force(ing) the US hand". Canada just wants to sell the oil. You see, there are 3 trillion barrels of the stuff up there. Besides, how difficult do you think it would be to abrogate NAFTA? It's a trade treaty. A piece of paper. The US and Canada have signed and abrogated trade treaties in the past and we are still friends.