Apple Music is in the same position as say Spotify on Google's platform as a third party media service. Is the implication that they should operate differently or be held to a different standard than Spotify?
I don't believe Apple has argued that collecting alternate payments in-app would be an undesirable for third party developers on the App Store - just that they don't wish to support it (for multiple reasons).
This also comes from Apple and Google having different starting points in their mobile platform - Apple already had a huge account and micropayment billing base for the iPhone by basing the App Store on the iTunes Store.