Huh? If you are talking last minute purchases, WGA wouldn't available anyway due to advance purchase requirements. Flat-tire rule is meant for limited usage in specific cases, it's not supposed to be used as a free same-day standby option. If WN thought free same-day standby was a good idea, they'd offer it.
I'm not even talking about last minute purchases. I'm talking about the OP and others booking in advance. When the OP bought the WGA, it may have been the last one in the fare bucket. Then, someone else came along looking to book and WGA was sold out...in advance (like, for instance, a month ahead of time). The other passenger may have gone ahead and booked with another airlines, because WGA was sold out and Anytime or BS was too expensive. Am I making sense?