A close analogy would be Apple owning every shopping mall in the world, charging shop owners annual rent AND taking 30% of their revenue. Also as part of their lease agreement, Apple forbids tenants from selling their products out of their own independent shop or even out of their own homes or websites. Could you imagine how much more expensive goods would be if shopping mall landlords took 30% off the top line of every tenant?
Of course you could say there's Google Play, but your Android app is a different product from your iPhone app. Also, Google Play conveniently has the exact same arrangement with the same 30% commission, so it's basically just a parallel universe where people are stuck in one or the other.
I don't know how they have gotten away with this for so long. Imagine if Microsoft took 30% of the revenue of every Windows app developer (of course beginning after their monopoly was already established)! They couldn't even get away with bundling a web browser into their operating system. What Apple is doing is far worse.