I feel for Spotify, but Apple did hit the nail on the head here. The App Store percentages have been in place long before Apple started competing with Spotify in streaming. If Spotify doesn't like the percentages, and can't make them work for their margins without increasing prices, they're able to pull their app from the store.
The problem is, they know they don't have the leverage to do that kind of thing, subscriptions would take a hit. I expect a fair portion of their customers are more loyal to their iPhone than to the music service, so if Spotify went away, many users may just flock on over to Apple Music instead of changing phones.
The reality is we've had a race to the bottom on the value of music content, and the margins are slim enough to where Apple's percentage cut really makes things tough for competition. Either you take a bath on each account (Good luck staying afloat), you pass along the markup (which means you can't compete price-wise), or you try to circumvent in-app purchases (which keeps your app from being approved, which we see here).
On one hand, I'd like to see them drop the baseline down to 15% to Apple, 85% to the developers, but on the other, in 2 more years you'll have companies with slimmer margins complaining about even giving up 15%. There will be complaints no matter the percentage used as long as someone else is taking a piece of the pie.
The root problem is Spotify's margins. Let's say Apple did give them a exemption and did 15% across the board for their subscribers. What happens when the labels go and stick it to Spotify, raising their rates again, and then Spotify can't afford Apple's 15% and we're back to square one with the complaining?
Ultimately, Apple's platform, Apple's rules. Don't like it, don't develop for it, make your own phone and app store. Go to Android and enjoy your 15% across the board. If you need Apple for your business model, then don't complain about the terms that you're willingly engaging in.
As an aside -- as a developer, I would like to see Apple adopt the 15% baseline like Google did. After a year is better than nothing of course, and I play by Apple's rules because iOS users buy at a better rate than on Android, and many of my company's clientele use iPhones, so it's where we need to be. I'm not all Pro-Apple in this, but I do think Spotify is being a bit disingenuous about the situation.