All Apple has to do to make it fair is to allow Spotify to mention/link to their subscription page. Spotify couldn't care less about Apple's iTunes billing, they'd be perfectly happy getting users to sign up through their site instead if Apple hadn't forced them to take out that piece.
Look, I'd really really like to hear your answer to this, because I'm curious. I don't think it's going to be sensible, but I'm open to changing my mind.
Let's say I'm an app developer. I have my own web site, of course, which advertises my game, but that's not where 99% of people find it from. They find it from review sites, or searching on the app store, or whatever. Now let's say this app is $4.99, or perhaps free-demo-$4.99-to-unlock-everything.
Now, let's say Spotify can get away with this. They can put their app up on the app store for free, and then put up splash screens inside the app that say 'click here to pay for our service on our web site'.
What's top stop me from putting up my app for $4.99 on my web site and putting up a splash screen saying to go there and unlock it there? I can use Apple's store for distribution for free, and my own 3% credit card processor for payments, and pocket $4.85 per purchase. Pretty significant savings for me!
Literally there is no difference here except that spotify is big and people like it, and I am small. You can say 'well theirs is a subscription' but what's to stop me from saying 'well mine is a subscription, for $4.99 you can subscribe to this app for 5 years'? And maybe offer a free extension if it's still around in that time? Trust me, if Apple let Spotify do this by making a subscription exception in the T&C, there would be a thousand lawyers licking their chops ready to defend me in court if I wanted to use them.
Perhaps your argument is that because spotify is available on other platforms, it's different? Well, there are ten thousand things that are available on the android store and the ios store. Are they different too? What about the finance app I have, which I bought from the company directly for the Mac and from the app store for iOS? Should I be able to buy the iOS and the Mac app as a bundle from them for one low, low price, and Apple doesn't get anything for distributing it although I'm making money off their distribution? What about if there's a half-assed web site that integrates with the iOS app. Perhaps the app is a game, and the web site organizes your saved games, and without it you can't save a game. I can sell a subscription to that, advertise it from inside the app, and collect the money. I get free distribution through the app store, and I can sell saved games and only get charged a credit card fee! It's like free money!
Apple is offering an amazing service to its developers: in exchange for them providing free apps to their community of users, Apple is providing curation, search, and distribution. I have some minor issues with the way they do it, but make no mistake: this costs Apple a huge amount of money, and for those free apps it earns them zero. What you are saying is 'because Apple does not charge developers for this amazing service unless the developer is making money, Apple should not charge them for this amazing service even if they are making money, unless they voluntarily choose to give Apple money.' You don't know that's what you're saying, because you don't know how fast a business would come out that would set up a subscription model, charge app developers 5%, and sell 'subscriptions' for apps for you. (Hell, I could write a site like that in maybe eight weeks, and I know a dozen people who would just looooove to 'disrupt' the iOS store.) And Apple would be stuck with the entire upkeep for the app store as overhead and no revenue whatsoever.