That's a good point.

The reason is nations are completely sovereign, where as
states/provinces/territories are not to various extents depending on
the country concerned and their constitution.

When dealing with sovereign nations, i.e. US and Canada, you can't
make a law but must have a treaty. That's why when I said earlier
that there should be a customs/immigration union between the two
nations, it would have to be negotiated among "equals" as opposed to
federal laws and what authority they have one other jurisdictions in
the same country.

This is spelled out specifically in the US Constitution in
the "Commerce Clouse".

Averill