Is this not the idea behind the Mac App Store?

Not really, no. Most of Apple's money comes from hardware sales.

“Services”, as apple like to call the heading they report App Store revenue under, is largely the fastest growing thing at Apple. While historically it’s true Apple has made its money on high margin hardware, the future growth seems much, much more reliant on services. I wouldn’t be so quick to discount the increasing importance of service revenue such as the app stores to Apple.