Operationally, an airline is always looking to off-load passengers who are not going to be on a plane. This may be because they have oversold the flight and are looking for no-show to make it work, or they may just want to reduce liability in case of weather or mechanical disruption.

If you buy a ticket to travel A-B-C and don't show up at A, an airline, even one as generous as Southwest in allowing hidden-city ticketing, is going to eagerly want to cancel your boarding pass at both A and B.

This is not an actual policy that I know of, so you may choose to wait until someone from Southwest comments in this thread. I am merely pointing out that it is natural for an airline to want to cancel your boarding pass (and any downstream boarding passes) at the earliest legal moment.