To clarify: on entry to the US, a visa-waiver national is given allowance for a 90 day stay (usually). If the traveller leaves the US, but remains in North America, and then re-enters the US, they won't give you an additional 90 days. Instead, they'll just treat it as a continuation of your initial 90 day authorization.

Note that it isn't illegal to enter the US for two months, and then spend two months in Canada. It's just that the US won't let you back in until you've left North America. How exactly (or even if) they know you've left North America, I have no idea.