Yes there was a big issue with that about 2-3 years ago....around the time the Cdn dollar was crap. You might remember a case where Chrysler announced that they would no longer allow their vehicle warranty plans to be transferred to other countries (or transferred without a fee). Honda did the same thing. Toyota thought better of this because it puts undue pressure on the customer who might just have been transferred there. (Kudo's Toyota!)
Anyways that practice was to hopefully stop the flow of vehicles from being purchased in Canada and shipped to the US. It was virtual piracy - and lots of little companies spawned just to do this practice (and a few dealers I might add

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If memory serves me correct, Toyota was having a huge supply problem with Sienna in the US...so people were pillaging the Canadian supply as well. Even with the exchange, tax, cost to convert the dash, etc. it was still worth it in the end.
Long ans short - the Canadian market cannot support the same price point per vehicle than in the US. So cars are usually cheaper here.