I think a lot of people are missing the point here. Many keep making metaphors about buying something in a store or on Amazon and the retailer taking a cut. That's understandable...it's not really what's happening here, though. Imagine going to Walmart and buying a product that requires a subscription to work--an Abobe CC Suite disk, for example. Now imagine Walmart requiring that...
1. You MUST activate the disk with special Walmart kiosks in store. Adobe is not allowed to provide any instructions for activating at home.
and...
2. Each month you pay Adobe for your subscription, Walmart gets 30% of that subscription.
That would be absolutely ludicrous; that is what apple is doing. Walmart has a loyal customer base of millions and has spent the countless prerequisite hours and dollars necessary to create an infrastructure that draws people in and keeps them coming back, but no one would ever suggest their doing so entitles them to a profit of subscription services just because the initial product was purchased in one of their stores.
You want to take a cut of a product bought at the store (an Adobe CC disk or the Spotify app)? That's understandable. You want to take a cut of a service that product provides and you have no part in providing? That's ridiculous.
It's a nice attempt, but it's not exactly what happens here. Rather than Walmart, you should imagine the following...
1. You buy a new, luxurious house. You've paid a hefty sum for it, but it's worth it. The developer has built it using some of the best technology available on the market. Because the house is so great, they have you 5 years warranty on it, so when the doors start to creek or the drain is getting clogged someone comes in and does the maintainance for you, free of charge.
2. You love living in your new house, but obviously, you don't just sit arround in it doing nothing. You want to "live" in it, sleep, eat, watch TV, listen to music and so on. The developer has a division that serves other markets than housing, such as utilities, entertaiment, clothing, even take away restaurants. You can get your electricity, Internet, TV subscription even your food from a company affiliated with the developer. But you don't want to. You prefer ta TV subscription from another provider and you don't fancy the developer's cuisine that much either. So you order a different restaurant on your lindline and order your food in from a different take-away, but when it arrives the pizza boy wants to charge you a mark-up of 30% because your developer says charges extra if a) you want to eat the food at home, which they built; and b) you used the landline which they set up. You refuse, this is of no concern to you. The pizza boy gives in and his company soon goes out of business, because your home is not the only one in the vicinity built by that paritcular developer. And it's only the start. You wnat to order some new clothes online? You have to order them through the developer's affiliated delivery firm, or else pay a markup of 30%. Not happy about it. Well, you can always wear the clothes at your neighbour's place, but at home, you need to walk around naked, because the developer build the house and we all know that whoever builds the house calls the tune. No need to worry, it's a free country, as a consumer, you always have a choice. So you either buy all your stuff from/through the developers afiliated companies, or you pay a 30% markup or otherwise see one independent local goods and service provider close their business after another. Or else, if you don't like this arrangement you can always go and get yourself a new home. It is a free country, after all.
The developer is obviously Apple, and the home in this story is your phone. You - the customer - have already paid for your phone. You paid only once for it, but you still enjoy a few years of "free" (or rather - pre-paid) mainteinance and support. You've paid to Apple for developing it, including the utility access points. In fact you bought the house and paid for it through the nose precisely because it had a lot of the amenities and utilities. So, unless you want to tell me that your landlord or housing developer has the right to charge you 30% on every morcel of food, clothing and whatnot that you bought though a 3rd party but consume the house owned/built by them, then you should not support apple for charging you (yes you, not Spotify, because they either have to pass these cost on to you or go out of business) for every service you consume "through their appstore".