There was distribution that costed precisely $0 in middle man fees. Software downloaded from the web (if you already have the audience, the distribution costs are marginal on the web).
Then Apple decided to cut it out. And also cripple web apps so they can't even be alternatives properly. Can you build a Mail app on the web? No. Apple didn't add the required features to Safari that developers could build on top of. And Chrome/Firefox/Edge did add them and they work. However on iOS, Apple does not allow them, instead they're forced to use Safari under the hood with Chrome/Google branding sprinkled on top.
Tell me how much publishers pay to distribute on Windows from their websites? Or the WebApps that don't need to be installed? Apple provides little value as a middle man.
The just wanted to be parasite middlemen and they don't deserve to be. They're the mafia.
Go for them under anti-competition clauses. And if there is indication that they have colluded, start trust busting on both. Google is already not trustworthy. They colluded with Facebook already. And Apple colluded to lower wages with other tech companies.
It is entirely reasonable to think that this is anti-competitive and hinders innovation.
Even with ePubs book publishers are likely to take 50—75%. Print books the publishers still take 85% and higher.
Now that is a tax. But I would not call it collusion.