Years ago, I think the problem was just the opposite; new cars actually being cheaper in Canada. And you could buy a car there if you were an American, but the warranty would not be honored if you brought it back to the States.
According to the article, the warranty issue may still exist as well as a lack of financing options .
In some cases, dealers said that vehicles purchased in the U.S. and brought across the border would not be protected by warranty.
Rogers says that in his experience bringing cars across the border, this has not been the case. “Some manufacturers do not honour the warranty in Canada, but very few.”
(edit)
Some dealers were more accommodating to Canadian consumers, though this varied by location. Ford, Kia and Nissan all agreed to sell new cars to Canadian customers, though the Kia and Nissan dealerships said they could not guarantee their warranty would be valid in Canada. And in all cases, the vehicles have to be paid up for upfront, as financing isn't available.