Understanding that SWA is an extreme example, but this summer I purchased RT tickets for $66.40. Four of those cost $265.60. Besides Ebay, there's at least one website that's buying Reward tickets at $300, leaving a nearly $35 "profit". If you didn't have to ride, SWA short flights would be sold out (but empty) and long hauls would be full with free riders (remember, SWA has no reward ticket capicity controls).

Even more, 25 of these flights would equal a Companion Pass, meaning that a FTer could "earn" $875 (if he sold his Rewards tickets) AND get a one year pass! Given sales and bonus mile programs, similar situations could occur on other airlines.

That's one reason why you have to fly, not just buy.

(For the record, the rules say that SWA Rapid Rewards are fully transferrable, but may NOT be sold.)