john wrote:

I think it is TOTALLY unfair for SWA to stop a passenger on the RETURN
trip and tell the passenger they must buy two seats to return home.

If SWA did not demand that the passenger buy two seats before he
boarded the first leg of his trip then they should not be able to make
that demand on the return trip.

What if the first flight wasn't full and the return flight was full?
If the flight isn't full you don't pay for the second seat.