Undoubtedly the Flying Tigerswere amongst the greatest Fighter Pilots in the history of American Aviation…But:
Seeing that they were all civilians acting pretty much on their own behalf by flying for the Chinese without US Permission, couldn’t the US have charged them with murder for the unauthorized killing of every Japanese pilot they shot down if it wanted to.
Don’t think so. They may have been American nationals, but they were fighting under the Chinese flag. They would have been treated as Chinese soldiers under Geneva Convention rules upon capture.
This is hardly theoretical, even today. Foreign nationals routinely fight for their adopted country, and the United States has many resident and non-resident aliens in its armed forces.
When I got out of the Navy in 1998, my last ship had about fifteen crewmembers who weren’t citizens, out of a crew of about 280. They came from various Latin American countries and from the Philippines, with one crew member being a native Scot.
". . . It took direct personal intervention from President Roosevelt to pry the pilots and ground crews from the Army and Navy. On April 15, 1941, an unpublished executive order went out under his signature, authorizing reserve officer and enlisted men to resign from the Army Air Corps, Naval and Marine air services for the purpose of joining the American Volunteer Group in China.
Orders went out to all military air fields, signed by Secretary Knox and General Arnold, authorizing bearers of certain letters freedom of the post, including permission to talk with all personnel . . ."
Since that is quoted from the cite you listed, I’d say that you answered your own question. 
They had permission.
If I were a civilian who went on a killing spree in France, or any other country outside the US, I would would not be subject to punishment in US courts except in a few limited circumstances – killing a US diplomat, for example. If I stick to killing foreign nationals, and kill in a manner that doesn’t get me defined as an international terrorist, or pirate, I can’t be prosecuted in the US.
However, if I return to the US and get picked up for some reason by the cops, I may be extradited by the US government back to the country where I did my killing, to face charges there. We have two-way treaties with most foreign governments to allow for this.
Ironically, in some ways Americans are safer killing in America, and then fleeing to another country. Many countries have very firm positions against the death penalty, and will refuse to extradite people who would face the death penalty after being returned for trial. What happens to those people varies country to country.
IIRC the Flying tigers began flying freight into China. They armed their planes because the japs were shooting them down. At least thats what I remember from reading “Claire Chanult of the Flying tigers” when I was in jr/or high school.Which has been a long time ago.
That’s totally wrong. The freight business was a post-war venture of veteran Flying Tigers combat pilots.
Based on everything I’ve read all pilots were U.S. Army Air Force, U.S. Marine, or U.S. Navy pilots on ‘leave’ with a guarantee to return to their previous duty with no loss of seniority or privilege when their time with the American Volunteer Group (Flying Tigers) was done. By the time they started flying combat the U.S. was at war with Japan so even though they weren’t Chinese they were fighting a declared war under the auspices of an ally, just like many of the first American fighter pilots fighting against Germany were flying under the control of the British since the U.S. didn’t have command and control in place (or supply chains, maintenance ability, etc.). The timing was pure chance but they could not be called “illegal belligerents”.
IIRC, they were also mercenaries. Getting paid (maybe a bonus) for every confirmed “kill.”
I knew a flying tiger vet when I was about 10. I was enamoured with WW2 and fighter planes. The vet did not want to talk about his experience in the least.
Boy, that wasn’t my experience. My day was a VP at Flying Tiger in LA during the 70s and I got to meet some of the original flyers a few times.
To this particular wide-eyed 7 year old they certainly talked it up. All bang! crash! swoosh! and the like. I enjoyed it immensely. And one of them gave me a model of one of the fighters that was in the lobby of the building. I lost it someplace but it was a treasured possession through my boyhood.