Right there.
"Developers offering products within another category of app downloaded on Google Play must use Google Play In-app Billing as the method of payment"
The exceptions are for physical things, and for things that can be used outside of the app. You can't use a Spotify subscription outside of Spotify.
> Payment is for digital content that may be consumed outside of the app itself (e.g. songs that can be played on other music players).
As Spotify music can be used outside the Goole Play app, Google doesn't take a cute.
The same reason I can buy Kindle ebooks within the Android app
