Yes, they're paying for the infrastructure. That fact is not lost on me.

But think about the real costs. Assuming that they have a nice scalable multi-tenant design, a ghost town is not going to have a lot of content or users, which means only a small hit on their database. Yes, tons of ghost towns add up to significant costs, but that can be easily fixed with some simple rules around inactivity over a set period of time.

Wayne Gretzky famously said "You miss 100% of the shots you never take." StackExchange is taking a 'rifle' approach. More power to them. My own personal opinion is that a 'shotgun' approach could result in more successful sites than the one they're taking. Just my 2 cents.