Users perceive the app store as giving a level of curation and security. If it became a list of "all Mac software that exists," and one of those turned out to be dodgy, Apple's image would take a real hit.

Also, what would listing software that hasn't been through the submissions process actually mean? That the app is verified by Apple? That going to the site and putting your credit card number into the shopping cart is guaranteed to be secure? That other things that haven't been listed yet are not as favoured in some way?

I'm not saying I like the app store model, or that they can't have a second tier of lower-scrutiny apps. I do think it would need a lot of thought and care, however.