I always thought it had something to do with AA flying out of T3 and if there was an overbooked flight it would be easy for pax to get between counters, seeing that AA only has one direct LHR - MIA service at the moment.
Why aren't all the BA USA flights from there you may ask, well I just assume that as there are lots of NYC flights etc each day, they can always get you on another flight.
Just a theory!!