You buy a Nespresso machine on Amazon. Some amount of the purchase price goes to Amazon for facilitating the transaction and delivering it to you, some goes to Nespresso for actually making the device. Cool. Then you get a pod subscription from Nespresso – let's say there is a touch screen on the coffee machine itself where you enter your details to subscribe. Now, Nespresso ships pods to your house every month. Amazon then says that because the machine was originally bought on Amazon, they are entitled to a 30% cut of that ongoing subscription price, even though the subscription is neither facilitated nor fulfilled by Amazon.
I think we can all agree that would be ridiculous.
That is what Apple is doing.