It isn't, because apparently you cannot advertise payment options in any way ... so a link to a web page where you can buy storage or where you can purchase features and so on is not allowed. Even though the app itself is just a gateway to an online service that exists outside the iTunes Store. And even though you make it impossible for the user to purchase anything by using said device, telling them something like "go to a desktop to make purchases".
I have Google Drive installed on my iPad. In my profile it does say how much storage I have left, but provides absolutely no link or hint on if I can or how I could increase that storage.
Right - the line between "Advertising" and "Purchasing" is pretty gray, particularly with in situ web browsing - what would happen if I clicked on a link and it brought me to an in-app web-kit interface?
Apple just makes it clear that if you want to purchase, advertise purchasing, or in any way try and extra hard $$$ from users through a native-client, then Apple gets 30%.
This is the only way their store will continue to work - any other approach will result in people gaming the system, and, instead of selling a $50 copy of Omnifocus, Omni will sell it for $5, but then have a "link" to Omnifocus's sync service for $45 for a three-year membership.
I agree with the OP up top - I can think of no way Apple can continue to run a store without toeing a hard-line on this.
Such arguments make sense only if the iTunes Store wasn't the only way to distribute apps on iOS.
I do hope they continue doing whatever they do, because such measures are really indefensible no matter how you try to rationalize it and this way developers and consumers alike are waking up to the fact that walled gardens are really a bad idea that will hurt us in the long term much more than anything Apple ever brought to the table and that the open web, for all its warts, is a much better platform.
I have Google Drive installed on my iPad. In my profile it does say how much storage I have left, but provides absolutely no link or hint on if I can or how I could increase that storage.