Not at all a question of ethics.
Ethics is a morality specific to an industry or some such thing where there is a different set of right and wrong applied due to some specific set of circumstances.
Executives & politicians have a fiduciary responsibility to act in the best interests of the business, not themselves (ummm, I know right, try not to laugh). Dr's need to put a patient above themselves, their research and their facilities in a very particular way. Teachers and child caregivers have a seperate set of obligations that apply because they have so much influence and control.
Never have I ever climbed onto an airplane as a passenger and felt so drunk with power that it seemed professional ethics should apply in any kind of way. Now do we need to behave etc, yes by law we do, but that isn't ethics, that's just rank and file applies to everyone law of don't be disruptive or visit the clinker
Credit could be a tax entanglement, maybe, but I'd suspect it is only income when and if you ever use the difference. When you get the credit it is utterly useless until it is used by only you at a very particular business, for a very particular thing (can't buy food or perks with it) in a very particular time-frame so it could be easily lost without use. I'd imagine a CPA could easily argue something for you because it's not a benefit until it is a benefit, and it does you no good sitting unused as a tied up liability to the airline, so it isn't even the same as money until it is used as money. Now, Gov mave have its own ideas but it's a bit muddy if it is left unused, I suspect this is why refunds are in the form of credit in the first place, the airlines have probably been down this road before and this was their solution.