I doubt they care much about advance planning of award tickets - they allow a few award seats on each flight, but don't care if they they are booked way in advance or the day before as it doesn't affect the revenue bookings (yes, there are scenarious where it will, but if there are award seats left 3 days out, it isn't going to have much impact on revenue bookings).

I think this is all about revenue generation - you waited until the last minute to book, and therefore we should charge you. I'm sure if they could charge $6 for a beer if you didn't have exact change, they would.