Also, if BA wants to attract members in the US, it is very much easier to earn status with AA when flying economy, but the extra TPs from first class travel make it easier to earn status with BA when flying at the front.

Someone who flies 25k miles a year in first would be ruby on AA but emerald on BA (assuming an average flight length of 1k and if they also met the qualifying flights).

So there are possibly good marketing reasons - attracting premium US-based customers who have to fly at long haul at least once (with connecting flights) on BA metal or codeshare, and are quite likely to do so in a premium cabin - as well as their associated high level of credit card spend.