It seems to me that Apple have rules, and Epic broke them and it's that simple. But if you consider that a small dev or an indie gets real value out of being able to use the App store, it basically makes the app known, and handles all the eCommerce, payments, obviously those services have a value. What happens though when you turn over $100Ks, or $100Ms and so on? Clearly the value of any positioning and services is then far less important - they we're probably paid for hundreds of times over already.
Various tweets and posts suggest that Epic has made this into a crusade for the benefit of all after initially trying to negotiate a different deal for themselves.
Obviously Epic is doing very very well with fortnite, and making a huge amount of money, but imagine if it were your company. It is clearly unreasonable for Apple to demand 30% across the board on the basis of fair-play. Yes, this goes against there being a level-playing field for the little guy, but Epic don't need advertising,nor payment processing, nor would they really need the App Store access at all if there were a reasonable alternative (or if they were allowed to supply their own alternative), so in this case, all developers deserve there to be some scalability in Apple's pricing. i.e. if you accept that Apple deserve a cut of what goes through their store, and you accept that bigger players actually require less from Apple, shouldn't apple charge a lower percentage as revenues increase? Perhaps Epic would have paid 30% on the first $50M or whatever, but only 10% on the rest say?
I can't say what's acceptable to both parties, but at the moment it's being painted as Apple takes 30% of your entire business on their network and that seems steep when you get into the big numbers.
Also consider if there were to be an alternative app store via another organisation than Apple - presumably unless Apple are ordered to allow this free of charge, there would clearly have to be a different tariff in place than 30% of all revenue - an alternative app-store couldn't operate under those terms.