Totally incorrect, unless it was a codeshare.

Legally the carrier that's on the ticket is the one responsible for the transportation; AA in this instance is just an agent. AA even reminds you of this in the Conditions of Carriage: "American will act as an agent to issue tickets, check baggage, and book reservations for transportation via other carriers which have interline agreements with American." Unless it's booked as an AA codeshare, BA is the one that has to find alternative transportation if they cancel their flight.

Having said that, AA cost-cutting has not (yet!) eliminated the practice of going beyond their requirements and assisting passengers with rebookings and new ticketing. Furthermore, given AA and BA's close ties and the high number of AA/BA codeshares, I expect AA to have some formal policy/processes in place.

You can monitor BA's policies at http://www.britishairways.com/travel.../public/en_gb; they are updated frequently.