In Canada, hockey is the more important sport. What you said makes no sense given your reasons. There's no "America's team" unless they are facing off against a Canadian team in the finals. That's a fact. It seems silly to me that US fans would cheer "USA" when they are facing the Blue Jays or a Canadian team in hockey, but at the same time it goes to show what they are thinking. You never hear us go "CAN NA DA!"
Because there are less Canadian teams than American teams in the NHL, Canadian teams are seen as "Canadian" teams by Americans. It's not the Canucks, Flames, Oilers, Jets, Canadiens, Leafs. For Americans it always end up being "us vs them." So make no mistake, when it's a Canadian team vs a US team, there is a "Canada's team." There is every reason to rally around a national team. Who do you think Americans would cheer for if god forbid it is the Canadiens vs the Coyotes in the final? They will rally behind the Coyotes simply because they are perceived as an American team.
It's human nature. When a certain group discriminates against you you come together and fight back.