I believe you're arguing in bad faith. Are you just becoming aware of these companies 30% cuts? Why were you calling Apple a monopoly if you think this is a cartel issue? What evidence do you have of cartel behavior other than that people followed suite in setting rates?
You shouldn't make that assumption without significantly more proof. Why can't it be both? In the App space Apple has the dominant position in a duopoly, and in the software distribution space more broadly there is cartel-like behavior occurring regarding commissions because distribution has become a major chokepoint (I'm guessing thanks to DRM, but that's probably oversimplifying).
