Paul Graham nails it:

They treat iPhone apps the way they treat the music they sell through iTunes. Apple is the channel; they own the user; if you want to reach users, you do it on their terms. The record labels agreed, reluctantly. But this model doesn’t work for software. It doesn’t work for an intermediary to own the user. The software business learned that in the early 1980s, when companies like VisiCorp showed that although the words “software” and “publisher” fit together, the underlying concepts don’t. Software isn’t like music or books. It’s too complicated for a third party to act as an intermediary between developer and user. And yet that’s what Apple is trying to be with the App Store: a software publisher. And a particularly overreaching one at that, with fussy tastes and a rigidly enforced house style.