I think what happens is that they still refund the money to the closed account. If the account number was changed due to fraud, it should still accept returns and they will just post to the new account number. If the account is truly closed, it should still accept a credit and the bank will eventually send a refund check to the cardholder (assuming that there isn't an outstanding balance on the account). In that case, the cardholder can generally request a check be sent out sooner if they want the money right away.

BTW, in both the "closed account" and "for tax reasons" situations, airlines are going to make it very difficult to get a refund to a form of payment other than that originally used for the purchase. If they were to make it at all easy, then it would be too easy for employees whose employers purchased refundable tickets for them to turn those into cash in their personal pockets. Another reason is that if it were at all easy to do, it would be the next sensation for the bottom-feeders in the frequent flyer mile ecosystem, somewhat akin to the dollar coins from the Mint: people would purchase expensive refundable tickets on mileage-earning cards, then refund them to standard cards, keeping the miles and costing the airlines the difference in discount rate.