Developers already pay $100/year for their developer licence. They also need to buy a Mac to develop in Xcode. And usually they also buy products such as iPhones and iPads to test their products. That 30% is added on top of that.
And THAT is the problem. It's called monopoly.
If tomorrow, Apple decides the new price is 90% instead of 30%, we'll all have to swallow the pill or up the price for the customers, so we can still get as much revenue. The other solution is to implement direct payment solutions, which will make Apple automatically pull or reject our app.
Luckily, on the Mac, on Windows and even on Android, there are still alternatives to App Stores, which enables us to lose just a few cents per transaction as opposed to a whopping 30%.