Probably the same way folks try to justify selling a DVD that only plays on North American DVD players. Generally people substitute the phrase "it isn't fair when ..." when they meant to say "I don't like it when ..."

I expect that the airlines will be unsuccessful in their suit but will get the FAA to approve a fee that would be charged if a passenger did not make the complete flight. In the back rooms they will tell the FAA its for passenger safety, after all its important in an emergency to know who is really on the plane, and people who get off early are putting first responders in harms way as they will go looking for people who aren't actually on the plane. They will argue they need a way to discourage that behavior and one way to do that is to charge a hefty "early exit" fee should a passenger depart the route mid-route. They would of course be open to discussions if it was for a legitimate emergency like you had to get back home or something.

And then there will be a $250 "early exit" fee and this web site will be toast and the airlines will be happy again.