I do believe that is their line of work. They sell seating. So if you need more than one buy two. No problem! Do you think that if you were the last customer at a bank, for the day, you should get all the left over money. Since no one was using it that night.
Thats akin to selling a guy a toaster, then not letting him leave the store and forcing him to by a warranty.
Their policy has to do with the inconvenience (if not pain) passengers suffer through when seated next to someone who should have purchased 2 seats.
In his case, they had unsold seats, he had already bought a ticket, he wasn't going to inconvenece anyone, they forced him to buy another ticket anyway.