Earlier quoted context omitted.
Not only that, "we charge 10% to the big monster and 30% to the little guy" is terrible PR against the amount of money they would generate from just the little guy. It's so bad that by that point they might as well just charge the lower rate to everybody. Especially when they're trying to stave off legislation.
Steam is doing this 30%-20%, and it seems to be working fine of them.. so far. I generally think volume discounts come a bit too close to price fixing, if proven in court. Granted, it's tough to prove, as we've seen in Intel vs. AMD which settled out of court AFAICR.
If they charge 30%, most small developers don't use it and then if you choose to pay, you get to be featured on a list without that many of your competitors. You're paying for exclusivity. Then you reach an equilibrium where small developers pay to be listed next to Valve's first party AAA titles, until there are enough of them that the exclusivity is sufficiently diluted to stop attracting more developers.
That doesn't apply to Apple because anyone who wants to reach iOS customers doesn't have any reasonable alternative way to do it, so there are millions rather than thousands of apps in the store and the exclusivity of being listed is already diluted to nothing. But they still charge the same rate.