It would have been helpful if the article had described in more detail just how goods destined for the US pass through Canada en route from the produced nation to the US.

Clearly, if such goods are commingled with (or become a component in the production of) Canadian goods en transit, then there is merit in the US request.  On the other hand, if the US simply wants Canada to do the heavy lifting as its surrogate, then I see no reason why Canada should assume this burden.

Am I missing something?