Who is the bad-guy here? (Both probably, but who is the worse bad-guy?) Obviously Apple's letter makes it seem as though they are being perfectly reasonable, but neither of the articles that I read have provided much analysis.
Apple's point of view is that they created the iPhone and spent a lot of money to build the app store, so they get to set the rules of the store. And further more, they treat every dev in the store exactly the same - whether you are Google or Spotify or an indie dev. Everyone pays the same fees to use the store. Since Spotify has decided to break the well-established store rules that apply to everyone, of course their app should be rejected. Apple is just applying their rules.
From that very micro point of view, Spotify is 100% wrong and the app should be rejected. Except that totally ignores the larger competition issue and conveniently forgets to mention that Apple itself isn't subject to it's own rules.
Spotify's point of view is that Apple's App Store is the ONLY way to sell apps to iOS users (to users who own their own devices!) so Apple has a de facto monopoly over iOS software sales. As such, they should by subject to antitrust law which provides limits on what a monopoly is allowed to do. Specifically, they say the rule that Spotify has to pay 30% of all subscription fees to Apple just to allow user sign-ups in their app is unfair. It's unfair because Apple has a directly competing product called Apple Music. Apple Music has an unbeatable advantage over Spotify on iOS because they don't have to pay 30% of subscription fees to a third party. In a low margin business, no one can possibly compete with Apple if they have a 30% price advantage.
I tend to agree with Spotify. Apple is trying to cripple their competition by giving themselves a huge, unfair price advantage by using their monopoly over iOS software sales. It would be fine for Apple to take a 30% subscription fee cut if Spotify chose to use Apple's subscription billing service. But it's not a choice - iOS apps aren't allow to use any other subscription billing service and aren't allowed to even mention other ways to sign up outside of the app. They can't even say the words "go to our website to sign up" without getting kicked out of the store. That seems evil to me. That's the crux of the issue. IMO, Apple created those censorship rules specifically to kill competition with their own products.
It's very similar to Yelp's complaints against Google. Yelp complains that even if they have the #1 search result for a restaurant, Google shows their own giant restaurant widget above all organic search results. You don't even see any organic search results "above the fold" anymore. Thus, it's impossible for Yelp to fairly compete with Google in the area of restaurant recommendations because Google controls the user's point of access.
The key factor in both the Apple and Google cases is if you believe the companies have a monopoly. Monopolies have more rules to follow than normal companies because they have the superpower to keep the market noncompetitive if they want to. Normal companies don't have that power in a fair market, so they are allowed to be more vicious in how they compete.