The real problem here occurs when you have hyper-competitive markets like song-streaming. Spotify, AM, Pandora, they all fight every contract and deal they make for fractions of pennies due to the micro-margins and huge throughput in their business models.
That's not really the problem; I mean yes, it is part of the problem, but it is not the core problem. If every vendor had to pay that percentage for Apple's payment processing, those companies could continue to fight for every cent, but they would be competing on equal terms. The problem is that Apple itself is in the music streaming market, and by requiring everyone else to use Apple's payment service (on which Apple makes a profit) instead of much cheaper third-party payment services, it is effectively granting Apple an unfair competitive advantage over everyone else in that space, because they can choose to not take the extra profit on the transactions themselves, and thus can offer their own music service for considerably less money than anyone else.
Now if Apple is willing to give 30% of their Apple Music revenue to music charities to level the playing field, that's fine (and musicians everywhere would love it if they did), but otherwise, what they're doing is blatantly anticompetitive, in addition to undeniably harming consumers by tricking iOS users into paying a higher price for the same goods and services solely because they happen to have downloaded the app and subscribed through the app rather than going to the company's website and buying a subscription before downloading the app. (BTW, I reject the entire notion that Apple "brought those customers to the table". Most people who get apps do so after hearing about them outside the App Store. Almost nobody I know has ever discovered anything in the App Store without searching for it; the store is just too big for that to be practical.)
The thing is, lots of folks pointed out all of these problems way back when Apple first announced their in-app purchase rules, and it has been an ongoing battle ever since. Eventually, it is going to come down to an antitrust suit, which Apple is likely to lose. And that will also be bad for consumers, because the app store rules do have a valid purpose—making it so that users don't have to give out their credit card to every random app that they do business with.
IMO, the only solution that won't harm consumers is for Apple to change their rules so that the "in-app purchases only" rule has an explicit exception for subscriptions to services that are also available on non-mobile platforms via a website. This would basically cover all the interesting edge cases, would cause Apple minimal financial impact in the long run, and would prevent this from turning into an ugly lawsuit where everyone loses.