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.
If the plane was booked full with no extra seats when he showed up, should they have told him he could not take the flight he had booked but would have to wait for the next plane and get 2 seats?
The airline is going to have to start asking a passengers weight and hight when they make the reservation and that is going to be invasion of privacy and people are going to be angry about that. What should the airline do?