Apple considered the App Store commission to be compensation for the value delivered by the entire iOS developer ecosystem, not just the mechanical/infrastructure parts of the app distribution process. It was a pretty good setup: aside from the $100/year membership fee, the charges scaled with revenue, which in most cases is a good approximation for the value provided (there were some edge cases where that falls apart, like digital content purchases). Unlike, say Microsoft, they didn't charge $250/seat/year for their full-fat IDE. The also haven't charged licencing fees for the SDK, like is common in the video game space.