It certainly wouldn't be the first time that AA lounge staff have done this. I have seen where people were "cut off" arbitrarily by certain lounge staff, while others were happy to continue serving. The US does have "dram shop" and "social host" laws so I'd assume that the AA bar manager is being mindful of the former, but a blanket ban on all BA passengers seems extreme. I agree that the most sensible option would be to limit those who have been drinking shots, or perhaps allow beer and wine only. If they do cut off the shot drinkers, they may just go somewhere else!