I think calling it 'stupid' is a little strong. There are hundreds of millions of iOS users, and many newspapers exist serving cities with smaller populations than that.
iOS is also a very different environment with regards to payment:
- On iOS the process of purchasing paid and free apps is the same: you type in your password.
- On the web the process of purchasing paid applications has built-in friction: 'free' is the default, and payment requires the user to trust the store, input card information etc.
There are more people on the web, but web users are less willing to pay for things.
I don't deny that a larger audience is better, but it adds overhead to support more platforms, so focusing on one can be an advantage. Marco, the creator of The Magazine (http://the-magazine.org), mentioned that he hasn't ruled out other platforms, but with limited resources it's easier to focus on iOS for now (he's the sole developer). Apparently after four issues he already has enough income to pay market rates per-word to writers and hire an editor.