If someone has paid for a seat, that seat should be theirs, regardless of if they choose to leave it vacant. Also, having paid for it, they should not be removed from it unless the airline has decided to refund them completely and provide some other benefit to have them volunteer to take a different flight. The airline should not be allowed to overbook their flights and rip people off. Clearly, the market has not been able to manage that given the United Fiasco and therefore I think it is within Congress’ purview to enact a law.