I think this falls in to the same category of the "flat tire rule", an unpublished policy like the free standby for delayed flights. I would consider a courtesy a occasional deviation for the normal rules for a specific situation, like the discount extended to my wife for rescheduling her flight to attend a funeral.

I know it seem trivial difference in wording but it also seems that if Southwest is charging a fee for a "courtesy" and that fee is consistent over time then it seems more like an unpublished policy.