Just to be clear, as others have pointed out, you didn't actually "book business on AA." You booked an AA codshare flight operated by BA. So it had an AA flight number but a BA aircraft. By regulation, this had to be clear when you made the reservation, and it probably was. Unless you used, say, a corporate travel agent over the phone, and the agent neglected to tell you. Then you'd have to take it up with the agent.