Do you know for a fact that airlines are responsible for paying additional fees to run a generator to provide A/C while the aircraft is at the gate? I would assume this is a sunk cost by the airline for whatever fee they pay the airport for operations. Most likely cause was an INOP generator, now whether or not AA chose to run the APU at the gate is a different story.

For ground truth, this is not unique to AA as I’ve sat on overly hot aircraft with multiple airlines, both US and foreign. In fact, it’s not unique to the airline industry. Most major hotel chains require motion and all doors/windows closed for the A/C to operate to save on energy, maintenance and of course money so there’s no reason to target AA in this particular instance.