If I walk into a music shop, are they obliged to tell me I can buy a CD cheaper elsewhere ?
I get where you are going, but I believe you're not making a good comparison. In the real, physical world, there is an extremely wide variety of shops. There will be a few bigger chains, but generally, not a single shop has an extreme amount of power.
If the creator of the CD (let's call him Spotify) doesn't agree with the terms of one shop (let's say a 15%-30% revenue cut), than the creator (=Spotify) could still sell his CD in dozens, hundreds or even thousands of different shops. He'll lose the ability to sell his CD in just one shop, but that won't materially impact his/her business.
On iPhone, there's just one gigantic shop (Apple's App Store). The creator of the same CD is now basically forced to go through this one shop, because if he doesn't than he'll directly lose access to at least hundreds of millions of customers.
So what does the creator of this CD do? He reluctantly agrees to the terms...
Suddenly, the shop owner (=Apple) also creates its own CD: different cover, same music. The product is 30% cheaper, because the shop owner obviously doesn't have to pay a 30% rate to himself. This means the original creator can't compete in a fair and equal way. He also is forbidden to even simply inform customers that there's still a small shop on an internet website, besides Apple's App Store, where you can get the CD for much cheaper.
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The discussion isn't really "should Spotify profit from Apple's ecosystem for free?". The discussion is about being able to compete.
Apple is no longer a small company. It's not a big company. It's a huge company. And the bigger you get, the more scrutinized you'll be for the actions you take.
So, for example. You could argue that the App Store is Apple's and no one else's. Apple build it, Apple is allowed to set the rules. But the App Store is the only way to reach billions of customers
and Apple actively forbids other companies from building alternative "app stores". That's not okay.
Basically, this all would be fine if Apple was a small company. It's not when you're the gateway to billions of people. And it certainly is not when you actively start to compete with other companies, while forcing your competitors to be at a disadvantage - because you (=Apple) set the rules.
No one would be batting an eye if Apple said "maintaining the App Store costs money, so you'd have to pay 5%, so we can properly maintain it" - 5% would probably be considered a fair percentage. No one would be batting an eye if Apple said "maintaining the App Store costs money, so you'd have to pay 75%, but we're allowing other 'app stores' to compete on our platform" - 75% is extreme, but there would be alternative app stores where people can go.