Hey Feliz:
I think that is true with auto's but I have not heard it happening with motorcycles.
The big 3 auto makers were selling cars in Canada for less money than in the US and with our much lower dollar at the time, it was easy to turn a profit by running cars over the border. (I think the climb of the Canadian dollar & fall of the US dollar has taken care of this). The big 3 stopped honoring warranty to try to stop this practise as you can imagin, a lot of US dealers were complaining.
From what I have seen, Mororcycles are much less expensive in the US, so I would see no reason for the bike dealers to be involved in not honoring warranty's.
I am not 100% sure about this though. If you were going to import a bike, I would make sure you checked this out with a US dealer.
I have a friend who imported a Guzzi into Canada from the US and had no problem with warrenty work.