If you don't fly the outbound the return does NOT cancel. The value of the outbound is reusable by the original traveler for up to a year from original purchase date. This feature is not like other airlines.
How confident are you on that one, nsx? The CoC explicitly says that they can do just that:
Failure to Occupy Space
If a Passenger fails to occupy a confirmed space on a flight and Carrier fails to receive notice of the cancellation of the reservation at least ten minutes prior to the flight‟s scheduled departure time, Carrier may, in its sole discretion, cancel all reservations held by such Passenger on continuing or return flight segments
http://www.southwest.com/assets/pdfs...f-carriage.pdf