Actually, Southwest's terms of service specifically allow it, but
their subsidiary AirTran does not. And they code share!

Is it fraudulent to get a meal deal at Taco Bell and throw away the chips?

Is it fraudulent to arrive in a city intending to fly back but then decide to stay?

Or maybe airline flying is like the all you can eat steak places when I was a kid in Texas. Eat it all or pay extra.

As to the OP, the one way fares are put in when competition, especially from LCC's, call for it. When they can, they go for higher OW fares and bundle the RT product to avoid competition. The more certain product you want -- the outbound flight -- is bundled with a later, less certain product, the return flight. It is easy to exceed even e cost of marked-up OW tickets with $200 change fees and increased base costs on the return under the Byzantine rules in effect.