michaelleung: Oh for crying out loud...
"You'll gain peace of mind knowing that we have your program stored and ready for you to download again [for FREE] at your convenience once your computer is up and running again."
So if you pay, you can download it again for free? Do you think I'm stupid?
As others have mentioned, this is because 3D Realms are using Digital River for software distribution, not because of something 3DR has deliberately done themselves.
You get this "download insurance" option not only with 3DR but with a lot of shareware providers who have opted for DR over another platform like Steam or Impulse. I'm fairly sure that EA used Digital River for their old store, too.
DR have been very long established (it has its roots in Walnut Creek, CDROM.com, and SImTel), and I assume that a low cost per download is why people keep signing up for it - even though it's us, the paying customers, who suffer as a result.