Now that airlines have wised up (for them) and moved from making reward seats available at 330 days and instead utilizing yield management, often times seats that are clearly going to be unsold (say a flight is only 35% full a week out) become available as rewards close in to the flight date. Those are great opportunities for airlines to clear mileage off the books without chancing losing a revenue opportunity. The close-in fee discourages travelers from helping the airline out in that regard.