I think it's less a question of the source code for the app needing to be available than it is that Apple wraps the app up in a container. Think of it like a filet mignon, a nice piece of steak wrapped up with a nasty greasy piece of bacon. The cow didn't put the pork into the steak, the restaurant did. I think the issue is that because Apple wrapped the app up in a sealed non-GPL compliant container, and then distributed it through the app store, without source code for the wrapper, they are the ones in violation of the GPL. Since the complete app is GPL code + Apple Wrapper, and since Apple is the one that adds the wrapper and sells the application...

I bought a paper cutter (the kind with the circular blade you slide up and down) the other day from Office Depot. I asked if they sold replacement blades. The girl behind the counter swore up and down that if I bought the $10.49 replacement plan and the blade dulled I could bring the thing back and get a replacement of the entire paper cutter. This was on a $50 item. I don't know if she was lieing or just dumb, but she swore up and down 3 times that was the policy. I laughed and refused to waste the money.

I'd put my money on both, lieing and dumb.