It's not just those two, it's standard practice: Audible, Google Play video, Amazon video, Amazon music, Playstation Video, Vimeo, Sky Store video, and YouTube Premium/Google Music (same subscription with different marketing).
Deezer music went another route: you can pay for your subscription through an iOS device, but you'll pay a higher price. [0] I'm surprised Apple permit this.
All this strikes me as a clear indication that Apple is asking for too high a cut.
I've never been tempted to use a mobile app to shop on, say, Amazon, but Apple are clearly ok with that experience being far better on Android than on iPhone.
> In my book, the problem is that you are not allowed to provide a link to your payment website inside your app.
Interesting.
The exact words used by the Audible app for iPhone: This app does not support purchasing content. Instead, add to your wishlist.
[0] https://support.deezer.com/hc/en-gb/articles/360000633989