Specifically, the "petite passenger" (and thanks, Bob Shallit of the Sacramento Bee, for providing us with her exact height and weight) who was bumped was flying standby. The teenager had a non-standby ticket (though not two, as Southwest usually demands for "customers of size"). Ordinarily, says Southwest spokeswoman Marilee McInnis, the airline would ask for volunteers before requiring a passenger to leave, but "airline personnel may have been influenced to choose a faster course of action to reduce embarrassment" for the teen. Not wanting to call undue attention to a teenager's size seems legitimate, but the "petite passenger" (who Shallit says "has asked to remain anonymous for fear some may regard her as insensitive") is annoyed, complaining, "It didn't seem right that I should have to leave to accommodate someone who had only paid for one seat."
When you fly standby, you can get bumped for a lot of reasons, and making room for larger passengers actually seems more legit to me than compensating for airlines' stupid practice of selling two tickets for the same seat. As McInnis says, Southwest "should have handled it better" — they could, for instance, have asked for volunteers without mentioning the teen's weight. Or they could decide that whatever money they save isn't worth their repeated PR nightmares, and just make their stupid seats bigger.
Bob Shallit: Petite Passenger Booted From Southwest Flight [Sacramento Bee]