<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by BigLar:
I worked for an airline 20 years ago and that was one of the first questions I asked. I never got a satisfactory answer either. 
They used to give these arcane scenarios whereby someone would buy a one-way and then, I dunno, buy another one or something, and screw up the accounting. Never did figure it out.
I think Southwest charged one-ways from the beginning and the World As We Know It didn't come to an end.</font>
Aside from the Internet one-way special, Southwest is just like the others now, all round trip other than full fare.
Here's the explanation: assuming everyone who qualifies for the cheaper round trip throws away the return portion, the people who do buy one way fares for a one way trip are those who don't meet the requirements for a cheap round trip.
Another thing to consider is that one of the restrictions for cheap fares is usually round trip required. This is done to enforce a minimum stay (one day, Saturday night, etc.) Obviously, one way fares have no minimum stay restriction.
So, if you don't meet any minimum stay requirement for a round trip, and you pay a higher fare, you often are actually buying two one-way fare bases, creating a round trip. I would guess that most one-way fares sold are actually part of a round trip.