im definetly sure they are just a rebel army without politicians voted in. without actual courts, without actual laws
remember the final S in ISIS = syria
ISIS is not the syrian government. never has been. its just a rebel group that just has "state" in the title, but doesnt have the government power to actually be a 'state' governer
the try to gain power using violence but that does not mean they get power
Go watch the 1959 movie, Journey to the Center of the Earth. Six went to the "center" of the earth. Two of them were crooks. When the big crook was caught, they held court. They could do this because it needed to be done, and there was nobody else around to do it. They could do it because that's the way British law works, and USA law, as well.
Court is made up of whichever people are forced to take part of it. Certainly pirates could have a court on their ship.
Further. If all the various governments of the world say that you, franky1, must die, but you are so strong that you, personally, conquer all the militaries of the whole world, who really is the government? After you protect yourself from death by conquering all the armies of the world, are you going to turn yourself over to their courts because you think that suddenly they will be nice to you?
Think a little deeper.