The 30% starts off as a publisher/retailer's slice of the fee, for which 30% isn't a bad fee. But Apple has extended its 30% take from "this is what we take for people who buy stuff on our store" to "this is what we take for any transactions within applications buy from our store." This is (very weakly!) justifiable if you're looking at microtransactions as buying-on-an-installment-plan, but Apple is going much further and demanding 30% on what's effectively regular credit card transactions.

And if people consider American credit cards' 3% fees to be borderline usurious, think how much outrage a credit card with 30% merchant fees would create. And that's basically what Apple is trying to push now, a credit card with 30% merchant fees. No wonder they're desperate to prohibit apps from telling people "you can buy it cheaper anywhere but Apple."