It's like this. When it comes to apps, Apple deserves their cut. They host the file, distribute it, and advertise it. They give developers a convenient, easily accessible way to sell their products. A 30% isn't that much to ask considering what they give in return.
But then you have programs such as Dropbox here, which only use the App Store as a gateway to the services they provide. Does Apple host the files you upload on servers they themselves provide? No. It's all Dropbox. They maintain the server themselves, do their own tech support, pay for the bandwidth to make sure their program works as intended. Why should Apple get a 30% cut of their monthly subscription fees? Because you use the app on an Apple product?
Hell, that'd be like Samsung demanding Comcast give them a 30% of your bill because you watch your shows on their TV.