Against this exact point, or rather guild tripping. I thought that you endorsed it by saying that Canada benefits from US imperialism. I am not familiar with Canadian nationalists claiming being oppressed by the US, but heard them bashing the US, which is a different thing. Who said that such bashing is undeserved?
Another point, the view that the North is rich because the South is poor is a bunch of crap not supported by any empirical evidence. However, it fits well the Calvinist guilt-ridden mind set that much of the left espouses. But like any religious belief, it is impossible to disprove it by factual evidence. But once you take that point of view, the downtrodden masses and countries can do no wrong - the evil resides solely in the North.
Wojtek