Developers offering products within another category of app downloaded from Google Play must use Google Play In-app Billing as the method of payment, except:
* where payment is solely for physical products; or
* where payment is for digital content that may be consumed outside of the app itself (e.g., buying songs that can be played on other music players).
Hmmm, except I can't play YouTube videos outside of YouTube and Play Music doesn't give me access to the raw MP3 files, therefore the second point wouldn't apply. I think the difference is that Google isn't proactively enforcing this rule so for example, the Kindle app can link to the Kindle store even though that's technically not allowed.
You can use the youtube app on your phone, or the browser on your laptop, or the youtube app on your tv. So it isn't content that can only be consumed in the app on your phone.
Developers offering products within another category of app downloaded from Google Play must use Google Play In-app Billing as the method of payment, except:
* where payment is solely for physical products; or
* where payment is for digital content that may be consumed outside of the app itself (e.g., buying songs that can be played on other music players).