Do we pretend the contract of carriage doesn't exist or that somehow it's not being agreed to when we purchase a ticket?
Rule 19 from United's (posted in the 2nd reply of this thread) clearly indicates the passenger is agreeing to pay the applicable fare for being returned on government order. And United isn't the only one with this language.
Delta in Rule 16b of their contract of carriage (
https://www.delta.com/us/en/legal/co...f-carriage-igr)
Similarly in AA's contract of carriage (
https://www.aa.com/content/images/ta...nal-tariff.pdf), rule 45b(2)
And no, that's not a mistake that the quotes are exactly the same, all the carriers use the exact same language with regards to this situation. I won't quote the United contract of carriage since it was previously posted, but yes it is the exact same language too if you don't want to look back at that post. My gut is this is standard language published by IATA and you will find it on every airline that has an international tariff.
Yes they could have a credit for the return, since this was hopefully a round trip, and that would likely be applied. But if the fare had a Saturday night stay or other minimum stay requirement, which a good majority of international round trip do unless they are unrestricted fares, that was no longer being met, the airline could come back on that front too. Basically the rule in each of them allows the airline, at their discretion, to charge the applicable fare for what is actually flown because that is what the passenger is agreeing to allow them to do by accepting the contract of carriage and choosing to fly on the airline.