A couple of things:
Canadians absolutely are cheering against the USA, not just for the Norwegians. I don't want to get into the root of it, but imo it isn't just the underdog, and it's kind of dumb. Blind mindless anti Americanism is annoying, and even though it isn't "real", it's still childish. I hope it's just because everyone keeps touting the Americans as the favourites, which frustrates people like me who think it's unfounded (or at least heavily exaggerated), but I think it's really more an outlet for frustration in other areas. In any case, I think people who think the cheering for the underdog thing is the only factor in the atmosphere last night are fooling themselves; it absolutely has something to do with it being the Americans on the other side. Of course the underdog will always get some support, but I'm willing to bet it won't be nearly so hearty when it's Finland vs Norway.