This is no different than any other aspect of pricing / service. Carriers look at competition in a given market and adjust. UA gives lounge access for its p.s. service (F for F and UC for C(J)) on LAX/SFO-JFK, but not on a 3-class 777 IAD-LAX. They've got serious competition for the NYC business, but largely none for the IAD business.

Same sort of things back when carriers served Y meals. Whether there was a meal was often linked to competition on the route more than time/distance.

AA has judged that it won't lose much business from this move, so it can cut costs and not affect reveue. This goes straight to the bottom line.