Big contradiction there!
Uber has basically zero marginal costs. They (usually) don’t employ drivers and don’t maintain fleets of cars. They just maintain an intermediary service - just like the App Store between consumers and developers. Or like booking.com - which just
notified its potential gatekeeper status to the EU.
Also note that Amazon does not have marginal costs when users buy ebooks - they pay royalties to the author/publisher as a percentage of the purchase price. Delivering one sold ebook to a customers does
not have low to zero marginal costs. Except the “delivery” of it, i.e, storing it on their server and transmitting it to a user’s device.
I think you and I have different understandings of what marginal costs entail.
Take amazon for example. Let's say someone purchases a $100 backpack online. It would not be feasible for Apple to take a 15% or 30% cut (or any cut) because that is coming from the retailer, not from Amazon's earnings directly. Nor is the retailer able to benefit from economies of scale because each bag sold this way incurs a variable cost (ie: you need to purchase 100 bags in order to sell 100 bags). It's not a situation where the more you sell, the cheaper each bag is by comparison.
You may be right in that Amazon is an intermediary service, but it is also a service that Apple is not able to bill directly, because there is no part of its costs structure that uses iTunes for billing, and iTunes is really the only way Apple can reliably calculate how much money any one developer is making through their platform, so again, it makes sense to bill developers a percentage of money paid through it.
Same for Uber. Apple would be billing the drivers, not the parent company itself. This is what I mean when I say these companies have high marginal costs - each transaction or trip is literally one car and one driver, and the costs associated with each trip scales in line with the number of trips being made.
With regards to ebooks, I am assuming that the author of the ebook owns the rights, and therefore gets to keep 100% of ebook sales, minus whatever Amazon's cut is. Let's say I have spent 1 month to write a book. That is a sunk cost, in that the book has been written regardless of whether I decide to market it or not. If I sell one book online for $5, I earn $5 (minus amazon's cut), at zero cost (remember, the book has already been written). If I sell 100 copies online, I earn $500 (minus Amazon's cut), at still zero cost (no need to print additional copies or keep them in stock). If I sell 1 million copies online... (you get the drift).
And assuming I leave the book up indefinitely, it would theoretically go on to generate an endless stream of revenue for me without me having to do anything (people could still be buying my ebook 10 years from now and I don't even have to worry about keeping it in stock or the pages turning yellow or dealing with shipping / printing costs).
This is why it makes sense (to me at least) to charge a fee for selling of digital copies of books but not physical copies, because of the cost structure involved. This also extends to say, digital media vs physical discs. Or to put it simply, something that is consumed on your device vs something that is delivered to your house. Which is what Apple has chosen to do here.
This is in contrast with a company like say, Netflix. The content has already been filmed (a fixed / sunk cost). The hosting and server costs are pennies compared to this. Because marginal costs are practically zero, each subscription sold is pure profit. Netflix's job is therefore to calculate how much to charge so as to arrive as the optimal, profit-maximising point. We can argue about whether Apple deserves a cut of subscriptions made via iTunes, but in the very least, it makes sense that Apple would bill them over say, a backpack sold via amazon because the former has zero marginal costs of production (and therefore can afford it) while the latter doesn't.
I guess Spotify is in a bit of a unique position because while it deals with digital goods, it has a cost structure that scales with subscriber count, meaning it is not able to benefit from zero distribution costs the same way a company like Netflix can. Perhaps it should be an exception to Apple's billing rules, or perhaps it's just evidence that music streaming is not a sustainable business model. Bear in mind that Apple isn't really collecting any money from Spotify (they migrated everyone off iTunes a few years ago), and pay Google nothing. So if after all this, Spotify still can't make a profit, I don't see how being able to bill customers directly in-app would help them even if Apple didn't charge them 15/30%. This is the part of Spotify's criticism of Apple that has always struck me as incongruous. Like yeah, Spotify's technically not wrong, but it also hasn't really applied to their business model in ages.
So yeah, tell me that my basic finance degree from 2 decades ago still means something? 😛