Not sure about canada since I don't go back there any more.
But, in the US...
The state has the right to establish hp limits. (in my opinion that is a proper state's right.)
With a state deciding the limit, each state can deal with their own situation... I doubt new york wants 800hp monsters running the capital, but in utah... with the salt flats... big hp is probably a good thing.
So I would say there each individual state of the US gets to establish limits.
edit:
btw, Im not sure about this, so you may want to go to the federal government site as well as any random state dmv site.