It may make sense for Apple as payment processor to take a cut, but not the 30% that they are for subscriptions. Subscriptions aren't actually adding features to the app, as demonstrated by the fact that you can subscribe outside the app (through Spotify's website), and still get the same content.
This is different than, say, my purchase of OmniFocus or another app. I'm not subscribing to anything, it's a one-time purchase enabling the app, perhaps more purchases later to enable more app features through in-app purchases.
The same problem, for me, exists with other subscriptions like, for instance, newspapers/magazines. Apple takes a big cut if I subscribe through the app, but going to their website I can subscribe and they retain more of the purchase price, and I can still access the content through the app.
TL;DR: Apple is acting as a payment processor and thinking they can get away with a 30% cut. Unlike apps which are hosted and vetted by Apple before making it into the app store, subscriptions to content are neither hosted nor vetted (beyond content type/suitability like porn being blocked - still?). The cost they impose on Spotify or any other subscription service is absurd.
That analogy is a bit tortured, but you get what I mean. Spotify, Netflix, Amazon, and other extremely popular services don't need that "we'll get you in front of customers" service that Apple touts as a reason why the 30% cut makes sense. And if you're a relatively unknown developer, you do get value from the App Store that you pay for with that 30% cut. However, I'd rather have Netflix than an iPhone. I'd have Netflix with or without an iPhone. Apple provides negligible value to Netflix or Spotify, in that while there are millions of customers of those companies using Apple devices, if Apple went away tomorrow, those customers would just buy a different phone and continue to subscribe to Netflix or Spotify.
I don't think there's anything necessarily anti-competitive to the level of thinking its legally actionable, but I certainly do find it a bit distasteful. The Kindle app is just better on my Android devices than it is on my iPad. It's a better user experience because there's no one putting arbitrary inconveniences in my way. And I dislike giving money to useless middle-men. I'm happy to pay Apple for the iPad -- they made a thing that I enjoy. I'm happy to pay Amazon for the incredible store and ebook infrastructure they've set up. I might like more of the money to go to the author, but at least Amazon is providing me some value in the transaction. Apple provides me nothing of value in that ebook transaction. They're just a leech.
Apple is big enough that billions can easily be insignificant. According to that article, they've paid out $15 billion total to developers. That means that Apple's cut has been about $6.5 billion. That's over a period of eight years, so somewhat less than $1 billion in revenue per year. Currently Apple is making $10-20 billion per quarter in profit, with yearly revenues beyond $200 billion.
Only jailbroken iPhones can install apps outside of the app store... or has things changed since last I checked?
It would be as if a product inside a Walmart demanded to be allowed to put up a display INSIDE Walmart of their product with a large sign saying instead of buying our mop for $13 here, come out to the parking lot and I'll sell it to you for $10.