These domains wouldn't be worth a penny if they came with a disclaimer that they can be deregistered or even taken over at any point. With a bit of luck, somebody could snatch popular last name domains and set up a catch-all mailserver.
People persist in "buying" digital movies e-books, and games despite the fact that they can disappear at any point, without warning, because the seller's license expired or they just feel like discontinuing the service.
They do have such a limitation, because of things like UDRP. It's just that there are maybe some forms of restriction on the processes by which domain names can be involuntarily transferred or seized. But it's a real possibility!