The problem is that Apple is double-dipping
Lots of people buy iPhones/iPads because various apps (e.g. Foreflight) only run on them. Thus, the App Store helps Apple sell devices.
They then have the gall to charge developers a large fee for their hard work selling Apple products.
5% or 10% might be overlooked, but 30% is ridiculous.
I'm an Android fanboi because of things like fdroid, which sells open source apps only, and compiles them from source itself, so you can look at the source of the app you're running. You can't get that with an Apple product because of their monopoly.