You have a good point. To make this change, the treaties would need to be renegotiated. That means the change needs to be retified by the Senate. Or was this part of another piece of legislation? It would be scary if Canada doesn't require passports and doesn't tell this to U.S. visitors that are unaware of the change. They let the Americans in, but the U.S. will refuse admission to returning Americans without a passport.
Once we require the passport to reenter, Canada and Mexico will require it to get into their countries.