True, AC -- and CP when it was around -- often matched lower fares between US and overseas points, since that is the market-to-market situation. How much traffic they attract is questionable, but such tarrifs are common in the business. You'll find most airlines offer cheaper international fares when you transit through their hub. BA has been accused of ripping off UK residents when it flies customers originating on the continent via LHR overseas at lower prices than UK-originating customers.

Unfare to Canadians? Maybe. But rates between Canada and these countries, non-stop, are also offered in competition with carriers from home country carriers as well as Canadian charter carriers. Pricing is set market by market based on demand or lack thereof. At present, there is a glut of seats on the NAtlantic as Americans are still not travelling abroad. This has depressed pricing ex-US cities. Canadian demand for seats ex-Canada is still strong enough that prices do not have to be depressed to these levels.