It makes more sense to me for other countries to not think about the specific countries/continents, but strikes me as odd for a Canadian to use American in that

It makes more sense to me for other countries to not think about the specific countries/continents, but strikes me as odd for a Canadian to use American in that context (I don't mean it as a criticism, though). Similarly, I would understand a non-brit getting confused between the terms English and British, but I would be very surprised to see a Scot doing that. And yes, USian doesn't sound or look right.