If it's an EY segment purchased on AA.com, then they're just a ticket agent, very little different from Expedia. Would Expedia be the one to call if EY canceled a flight that had been booked on their site? Likely not if one wanted to accomplish something.
This is an interesting point. When people report problems with AA and then mention that they've bought a ticket through a travel agent like Expedia, they're often told "why should AA help you--you are using a travel agent, they should be the one fixing your problem", yet here when AA is the ticketing agent, you suggest the opposite is the case. I'm not saying that your own positions are necessarily inconsistent, but if there's various scenarios where whoever sells the ticket should be on the hook, various scenarios where the issuing carrier is, and yet more scenarios where the operating carrier should be...well, it's no surprise that non-Flyertalkers end up confused when problems arise.