They don't. Seats are not airline tickets. As proof: airline tickets (or reimbursement) are guaranteed; seats are not
Cheers.
class of service is; when rebooked into X class, it is the passenger that is booked in that cabin; perhaps next is to offer a quick auction to others to offer the 1st class seat; based on the spurious argument given, this would seem to be just as valid as giving away to a passenger