Probably because Stihl Canada, and Stihl USA, (as well as Stihl Australia, &c.) are all separate companies marketing the (mostly) same goods and they each handle their own (but not any other company's) warranties?
I wouldn't be surprised if other major manufacturers of like equipment don't follow suit eventually. Unless, of course, they make equipment which is capable of being certified by all the disparate country's governmental standards, which might account for their inability to market a saw which utilizes all (if not most) of it's potential while sitting on the store shelf, which is what I understand Elux does.
I'd bet, though, that if you'd bought your Canadian 088 <i>as</i> a Canadian but moved to the States during the warranty period and brought it with you some leeway might be discovered.
Glen