Let's look at this from the other side of the equation. What do AC's customers get out of CO's route system? Connecting in EWR is no big deal; they can use JFK. IAH is so far from Canada it's practically in Mexico! Ah, but CLE is another story! Connecting there (CO's closest hub to Canada) makes sense for Canadians traveling to United States destinations.
While AC could always go to JFK, there wasn't much for them to connect to there. A few TATL flights or a CRJ to IAD isn't much good. Using CO as a connection in EWR literally opens up the world. I do agree that CLE is well positioned for connections as well, assuming that CO doesn't cut too many more flights there.
Originally Posted by
PSU Mudder
That seems to be the conventional wisdom, but I think I read somewhere on FT that US had to agree to let CO into *A, and that one of the conditions was that they themselves wouldn't get forced out. I'm speculating here, but it seems entirely reasonable that US, as a full *A member, would have some sort of veto power, especially with respect to new domestic entrants.
US won't be pushed out of the alliance, but they are not part of the revenue sharing/anti-trust immunity deal AFAIK.