Really, you think its worth their cut, just to operate the store? How much overhead would it take you to put up a web store? They only get away with it because they aren't Microsoft.
That's not all they do, though. They build the platform and heavily market it, too.
They do it only because they can get away with it.
Does your grocery store pay its suppliers, or does it merely skim and give the farmers what's left over?
If Apple provided no value in the equation you might have a stronger case, but as it is they handle an awful lot, from payment processing to marketing to distribution.
Apple is providing you an audience that's ready to buy, that has cash in hand, and all you have to do is bang out an app they'll hit the "Buy Now" button for. Sounds to me like developers are the ones doing the skimming, at least by your definition.
I'd like to see the headline: Apple leeches off of millions of developers.