Correct me if I'm mistaken, but I think this was a requirement for regulators, both in the EU and US, to approve the AA/BA partner agreement. IIRC, authorities were concerned about monopolistic practices ex. LHR to the US by a partnership between BA/AA and therefore required they give up an significant number of LHR slots (60+?) or reduce the potency of the partnership. Not surprisingly, they opted for the latter over the former and as such diluted FF benefits (as HeathrowGuy sights in his post) were a "weakened" component among others.