Spotify has to give the music labels and Apple a cut on iOS.
Apple only has to give music labels a cut.
On Android (and Windows/Mac) Spotify only has to give music labels a cut.
So here's the problem with what you and werks are saying, and ultimately where the DOJ argument falls flat.
The DOJ is arguing about behavior after the fact with Amazon. Fine, it looks shady, or does it?
So now let's look at 2011-2013. Spotify comes. Offers a service, you can sign up through the App Store. Oh look, you can also sign up for Google Music, Pandora, etc and pay through the App Store. You can also get a subscription to Entertainment Weekly, You can also buy coins for Candy Crush, etc... companies offer IAP, apple sees a cut.
But Apple releases it's own Music service, and now "well they have to pay apple for their competing service (which they had to do BEFORE it was a competing service), so that's not fair!!!"
So the problem eventually becomes, Apple is disallowed from creating ANY app or service that already exists on the App Store, or they are guilty of unfair competitive advantage. Or worse, another company can simply release a similar or identical service to apple on the app store, and then can complain about unfair competitive advantage.
It's a truly slippery slope.
Should we argue against Sony Microsoft and Nintendo who get their share for each game that is sold and not all is sent to the developer and publisher?
This is kind of my point. No one argues that they have a "competitive advantage" because they see additional revenue from sales off of PSN/XBLA/etc. They (along with App Store, along with Google Play, etc) are merely purchase facilitators.
Beyond that, the Amazon argument seems to be completely different than the Spotify argument. In the case of Spotify, Spotify is merely offering a subscription that apple gets a chunk of.
In the case of Amazon... arguably, apple did make it horrible for them, as they couldn't sell their 500K books in the app, unless they also listed all 500K books as IAP in the App Store. Beyond the 30% cut, the thought of managing that kind of data on the app store is simply a nightmare. I don't think Apple ever intended to make money off of Kindle books. I think they flat out were looking to make it unreasonable for Amazon to even SELL Kindle books on the app store.
This is not the same as Spotify at all. Spotify doesn't come down to "competitive disadvantage" at all. Even if Spotify just charge $9.99 on App Store (which they fucking should be doing), that 30% comes down to Apple facilitating the sale. If Spotify chooses, they can choose to not have Apple facilitate sales (like Netflix, etc). Does THAT potentially put them at a competitive disadvantage? Yes, but then it's there choice. Simply making less money than Apple per subscription ($7 vs $10) really has nothing at all to do with competition.