This has two effects: first, it makes certain products impossible to offer. The gross margin on an audiobook is 20%. Apple takes a 30% cut of sales. Try to sell audiobooks in an app, and you'll lose money on every sale. That's why non-Apple audiobook stores like Libro.fm and #Downpour require you to buy your books in a browser, which hamstrings them and gives Apple an unbeatable advantage (Apple doesn't charge itself 30% on every transaction, obviously).
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But at least you can buy audiobooks on Apple Books. There are plenty of services whose gross margins are lower than 30%. Apple's 30% #AppTax renders these unviable, and if Apple doesn't deign to offer its own in-house monopoly version, the service is simply unavailable as an iPhone app.
But that's not the only bad outcome. Some lucky service providers are able to pay the #AppleTax by gouging Apple's *customers*, raising prices to pay the danegeld. That's the second effect.
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