[threeseed]

You can tell people how to subscribe outside the app.

But you just can't use Apple's own platform against them.

[ClassyJacket]

You can't link, at least, to an external website to buy

https://www.pcworld.com/article/229880/Apple_Backs_Off_In_Ap...

[Jagat]

You can't even tell that such a thing exists. E.g., Audible only gives you an option to add to the wish list, and is mum about how to buy a book.

[hesarenu]

So in a way apple is forcing them.