My variation on Ockham's razor - the simplest answer is probably correct.

So you can go with a half page explanation about how airlines make mistakes, one hand washes the other, about how it's not really money but a perishable commodity, how they don't have any real objective value, about how airlines change rules on us so it is justified, etc.

OR

You can say - "I got something that doesn't belong to me, I should give it back"

If you choose the first, just admit you don't have saint-like ethics and spare us the convoluted justification.