This is a very interesting topic.
A few comments from a uk perspective. I am a surgeon in the NHS. So use to treating patients for “free”. We are regulated by the GMC who have published guidance for these issues.
If a call is put out a doctor is obliged morally and ethically to respond. If a doctor was not to respond and it later this was recognised by someone that doctor could be referred to the gmc for dismissal for failing in their duty as a doctor. That is unless the doctor is incapacitated or drunk.
The Good Samaritan act does protect us to a certain degree but if an error occurs then there is a chance of prosecution and/or lawsuit from patient and airline.
Airlines for years have used good faith and good will of doctors to provide free medical treatments on flights. It is only right that a reward/token of appreciation should be given to the doctor. I think that should be a free flight or upgrade. It is a small cost to the airline and a huge moral boost to the doctor.
It is not a simple black and white issue with many ethical and legal problems. I know of a few doctors that will have a couple of whiskies before boarding a plane so they are too drunk to treat someone on board.
Just because we are doctors by profession does not mean that society must expect us to be doctors every minute of our lives. There must be time for us to be humans enjoying some free time.
This could easily solved by airlines employing paramedics or nurses to be on board planes who could also be air stewards.