Edit: Relevant section:
"Developers . . . must use Google Play In-app Billing as the method of payment, except [when] . . . 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)"
Edit 2: To clarify, Spotify services are available outside of the app itself so I'm fairly certain they can avoid using Google Play's In-app Billing if they so desired. I have no idea if they actually do or not or if Google would actually have a problem with this or not.
[0] https://play.google.com/about/monetization-ads/
Edit: Relevant section: "Developers . . . must use Google Play In-app Billing as the method of payment, except [when] . . . 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)"
Edit 2: To clarify, Spotify services are available outside of the app itself so I'm fairly certain they can avoid using Google Play's In-app Billing if they so desired. I have no idea if they actually do or not or if Google would actually have a problem with this or not.