IMO, Southwest is probably one of the better airlines for a soldier to choose and not get gouged in the process. With respect to your comment, it depends on the meaning of the word "lose" to borrow a phrase.
As near as I can figure it, the difference between a seat sold 21 days in advance and one sold in the last 24 hours before flight is the "cost of money" or the interest that could be earned on the advance payment. At a generous 4% rate of return, this is about 65 cents on a $250 airfare.
Granted, there might be other savings having advanced headcount, etc., but I don't think anyone is saying to sell the seats below the airline's cost. The only "loss" I can see is the loss of gouging a last minute customer on a seat that could fly empty.
I remember belonging to a program Eastern Airlines had that offered seats at a deep discount 3 days prior to the flight. I think it was the "weekender" program and it made a lot of sense for them to fill unreserved seats for a great price yet still at a profit over the incremental costs of one more passenger. Isn't that more or less what the Mom's are asking for here?