Probably because they are roaming and would rack up the $$$.
On another note, The Apple Insider posted an article today regarding a possibility why the iPhone has not reached Canada yet.
You can read it at:
http://www.appleinsider.com/article..._data_rates_keeping_iphone_out_of_canada.html
EDIT: Forgot to add, if you want to sound like an American so people don't rip into you for your accent down there... instead of saying 'aboot' just switch it up with an 'aBOWWWt'... Got sick of my classmates ripping into me when I was there for school.
