@psuboater, I haven't flown internationally from SEA, that is why I said I don't know SEA's policy. But at SFO, have flown to Mexico (on United) and a couple Canadian airports, both on AC and UA, and left from domestic terminals. AC has several gates and a Maple Leaf Lounge in Terminal 2 at SFO, and that is a domestic terminal.
It seems the much longer haul flights need to use the International Terminal.