"The company has told some applications developers, including Sony, that they can no longer sell content, like e-books, within their apps, or let customers have access to purchases they have made outside the App Store."
The minute you allow any sort of third party authentication systems in an app, it becomes basically impossible to know whether or not someone is using those credentials to buy something on a website somewhere that enables access to more data in your app.
And they probably only really care about the big content sellers (Amazon, Google, etc.) so they wouldn't have that many apps to enforce it on.
As for the variety of different apps, Apple isn't exactly known for being consistent. I'm sure they could come up with different rules for subscription vs. a la carte downloads.