Frankly, we doubt Hippocrates spent much time worrying about billing thresholds. Of course a doctor will provide aid on a flight if a seriously ill patient needs help. But the airlines shouldn't be taking advantage of physicians' compassionate natures. In many cases, physicians on planes have been able to advise pilots that a passenger's condition is not life-threatening thus sparing the aircraft from making an emergency landing. This saves the airlines thousands of dollars.
Airlines hand out upgrade stickers for business class like lollipops to their most frequent flyers.
That is the least they should do for the physician when a passenger is ill enough that they request the assistance of a doctor. And if the doctor's work is especially onerous, they should do a lot more.