Originally Posted by D582

This has nothing to do with cabotage, which are rules governing the transportation of passengers and/or goods between two domestic points by a foreign carrier. AC cannot operate LAX-SFO, and cannot sell you LAX-LGA with a connection point in YYZ. That's cabotage.

The OP is asking why AC does not sell, on its own ticket stock and via its own website, UA flights for solely domestic US travel (or domestic flights within other countries on other airlines).

AC absolutely can do that using the fares filed by those carriers that it has an interline agreements with and where the fare in question does not have plating restrictions. They don't do it because they don't want to be a travel agent for other airlines as they would earn no (or very little) revenue from doing this but would incur all the costs of servicing the itinerary.

Originally Posted by RatherBeInYOW

Furthermore if AC did issue for example a UA-operated flight between EWR and SFO on 014 stock you wouldn't get any additional benefits over and above just buying the ticket from UA anyway adding your Aeroplan number to the reservation. No eUps because it is UA metal, your boarding priority, lounge access, etc. is solely based on whether you have *G (i.e. 50K+ on AC) or not and you're going to get credit for the SQM. The only thing you are really missing is getting credit for SQD.