IANAL but I’m not sure that’s always true, legally speaking. You may feel like you’ve been robbed, but whether an actual theft has been committed depends on the terms and conditions stated at the sale.
For example, I recently bought a gift certificate from a local business that’s currently not open, and the fine print says that if the business is unable to honor the gift certificate in the future then I’m not guaranteed my money back. Likewise, airline tickets may not be refundable if a flight is canceled due to circumstances beyond the airline’s control. So I don’t think it’s quite as simple as “if I paid for something and didn’t get it then they have to give my money back or it’s theft”.
Theft typically involves taking possession of property without consent and with the intention of not returning it.
It’s hard to argue that the airlines took money without consent, and there was never any intent to return it, nor was there any expectation that they would.
So that covers theft. Maybe what Atama is saying is that airlines have committed a form of fraud. But again, criminal fraud involves deliberate deception at the time of the financial transaction, which doesn’t seem likely.
There’s no crime committed, though there might certainly be civil remedy in the name of consumer protection. But my guess would be that if the airlines believe they can’t settle up somehow with a significant number of the people who have outstanding credits, then bankruptcy protection is probably going to be their remedy. Not everyone who’s owed money gets it back, unfortunately.