You want to outlaw price discrimination?
Obviously if they tried to charge 85% no one would actually be a developer and the App Store would crater. So they have a very strong incentive not to.
But from a legal point of view is there a reason why one number would be legal but another wouldn’t?
The only thing I can think of is a contract being determined to be unconscionable. Until they hit whatever threshold that is it seems like it’s completely up to them to set the terms at whatever they think enough developers will accept.
That's not really true. If the iPhone became a real monopoly in the US, for instance, with perhaps 98% marketshare (similar to Windows before Macs started seriously challenging them), then Apple really could charge 85%. What is anyone going to do about it? They'd have a choice of paying 85% to Apple so they can sell apps to 98% smartphone users (basically everyone), or not selling smartphone apps altogether and finding a new business strategy that probably doesn't involve making software for consumers at all (which admittedly, many developers would probably choose).
I think the missing piece that most people miss is that it is not illegal to have a monopoly. It's illegal to use your monopoly to bully others. Apple did no such thing.