It happened more often that you thought on various app stores. It could be pulled due to legal reasons or the company simply want to force you upgrade to newer version etc. The point is with physical media, you know you can always go back to the disc. With downloads, there is no guarantees even if you made a complete backup. We often upgrade computers. With the new PC, your old backup is useless.
That's the beauty of a Mac -- Time Machine backs up applications. When you migrate to a new computer you don't have to reinstall anything.