Greed? How much money does Apple make from the (mostly 15%, not 30%) fees? Unless you know exactly what Apple's profits from App Store commissions are, you're being upset just to be upset. You don't know if Apple makes anything on the App Store. It's likely they do, but we don't have that information (unless you work for Apple and happen to know). Maybe Apple's margin is only 5%. Is that too much? Maybe it's 70%. Is that too much? Are developers making money? Are developers making too much or too little money?
Why is 5% appropriate? That would likely lose Apple money -- it's expensive to run the App Store. How do I know 5% would lose them money? Apple's services gross (not even net) margins are about 70%. There is little chance Apple is making that much on the App Store but let's just assume the App Store is at a 70% margin. With an assumed 70% margin and a 30% fee (most developers are not paying that much), Apple gross "break even" point is a 9% fee. That's gross margin though and would in reality lose Apple money. If Apple mainly has 15% fees, Apple gross "break even" point is a 4.5% commission. Again, that's not net, so that level would lose Apple money.
What this means is 5% will almost certainly lose Apple money. 10% might only be a break even point. This means that Apple charging 15% to most developers and 30% to others, allows Apple to make some money.
Can Apple only charge a fee if it loses them money? Can Apple only break even? Is Apple only allowed a small margin? What is an appropriate margin for Apple? Why do you get to decide?