They wouldn't win in court then?
I don't know.
You said:
"does this not mean that the airlines have full control of their miles?" The answer is that they always owned the miles.
"i.e. that selling them would be illegal (and they could win in court)?" Selling miles is still not illegal. Your point about court is only relevant because you heard a heter that you can sell miles because the airline wouldn't win in court and don't have control of your miles. But that's based on a false premise - they
do have control of
THEIR miles. (Get it?)
I don't see what court has to do with it. There are other heterim around for other reasons.
"which would thus make it asur to sell?" If this was going on the 'it's illegal to sell,' then I would imagine it would be hard to find a Rabbi to matir selling if it actually became illegal. If it's going on, 'would the airlines win in court?" then I would say, once again, there are other heterim around. Look and ye shall find.