My understanding is the course “buys” a certain amount of tee times, so the “sell out” is a bit misleading.
I assume you pay your green fee (or at least a deposit) when you reserve the tee time? Could be that they like the one time influx of cash.
That being said, it gets attention every year, and they thrive off the exposure. They can’t just sit back like wild horse, so credit for thinking outside the box.
I like that method better at a place like Sweetens where you’re essentially buying a day pass, though.