But airlines are different. As soon as the flight takes off, the value of the empty seat is completely down the toilet, it has expired. (I know this applies to other things too, but airline seats are particularly dramatic in the high cost of everything involved)
But also, the original customer cannot show up half an hour after the flight departed, and still use the ticket. There is no possibility of it.
What if I am traveling with some else? We have adjacent seats. I missed the flight, my fare is non-refundable, and I did not cancel (so no hope of using the credit). My companion does makes the flight. My companion should be able to enjoy that space. It is unethical for the airline to do otherwise, and it should assume that I have a companion.
It's my space. I paid to keep it open. If the airline wants to use the space, then it ethically it needs to compensate me.