Amazon shouldn't, but then Amazon is the problem here. They could just go sell their Kindle and let Apple sell their iPad and iBook. iBook is not on Kindle store and Amazon is already given a separate platform to profit on while offering essentially the same service of channeling books to the readers.
Many people already pointed out profits from the app purchase do little to Apple's bottom line, so I don't see how they could simultaneously argue this is down to greed. In the end Apple has little incentives to let other business sit in the middle and feed from the ecosystem.
Not only does Apple's subscription cut at the worst make little difference to the consumers, probably both the consumers and the content originators lose out when there are more layers of middlemen profiting from the sale in the middle, and I don't know why people are clamoring for a return to that. The 30% cut is great for actual developers, just not middleman who used to live on that cut themselves.