When the first attempt at refuting a "completely wrong" position turns out to be completely flawed on the grounds of basic logic, it's time to speak up.

UA did not sell transportation from one Canadian origination to another Canadian destination. That would have implied that UA sold a YYZ-x/PIT-YVR through fare, which it did not. Nor can it. UA sold two transborder tickets. This pricing practice has been used by UA and AC (at least that I know of) for many years now.

If some people still have trouble accepting the fact that what appears as one ticket is actually (legally, especially from the point of view of regulators) two tickets, read this old thread from the AC forum. Of particular note are bocastephen's posts (#18, #22). On the other hand, the final post by californiadreamin' (#26) was unfortunately not refuted by anyone, as 49 U.S.C. § 41703 applies to a foreign aircraft flying between two domestic points, not passengers. Besides, U.S.-Canada bilateral has given U.S. and Canadian carriers greater flexibility that 49 U.S.C. § 41703 does not provide.

If anything can be gained from this and that old AC forum discussion, they show that situations like these are not clear cut cases of cabotage (imo it's a legal loophole). They also show that these instances are happening on both sides of the border. If they are indeed cabotage, you'd think that UA and AC would have been fined out of this pricing practice by now.