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Google admits Spotify pays no Play Store fees because of a secret deal | TechCrunch
(techcrunch.com)
This is a most excellent place for technology news and articles.
I'm gonna go out on a limb and say I think this is done to prevent anticompetitive issues. If Google were to profit off of both its own product (youtube / yt music) and also require its competitors to pay it a % of revenue, it would potentially open them up to more anticompetitive lawsuits.