It's not even about the 30%, it's about the monopoly power. Target can require 30% of my internet bill because I bought a router there, fine, I'll just go get another router from Walmart, Best Buy, Amazon, MicroCenter, or any other place that sells routers. There are choices, and the competition keeps bad behavior in check.Bad take, this and every one like it.
Imagine if target charged an extra 30% on top of your monthly internet bill, just because you bought your router at target; if Ford or BMW asked for 30% of every bit of gas you buy to fuel your car; if your TV's maker charged you 30% extra for your monthly TV, Netflix and Hulu subscriptions, just because they built the device you're using those services on.
Yes, hosting and providing data transfer (downloads) takes some resources, no it isn't free - but the upkeep is so, SO much cheaper than running a retail store. Retail means paying rent, employees, stocking product which (unlike digital goods) take up a lot of warehouse space, with remaining unsold stock (unlike digital goods) being a net loss for the store that has invested into that product.
Running the data centres surely makes up for the bulk of its total cost to Apple. The App Store also needs employees for auditing, moderation, new features and maintenance, but you might well be able to run that whole thing with as many people as a small handful of retail stores would need. Yet, they charge as much of a fee as any retailer.
With Apple App Store and Google Play Store, there is no choice. There is only one store for iPhone, and only one store for Android (effectively). And the stores do not compete, because I cannot choose to sell my app to iPhone users on the Google Play Store, and I cannot choose to sell my app to Android users on the Apple App Store. They are totally separate markets, and each market has one monopoly platform for app distribution. So they can do whatever they want with no competition to keep them in check. Today it's arbitrary 30% fees on transactions, but this fight is about preventing things from getting even worse.