Of course they should pay. This is Apple's store. These guys think they can use Apple's tools, etc to put a free app in the store and then make millions on the side. That's BS. If you use Apple's tools, free marketing in the form of the app store, then you owe Apple a royalty. Simple as that. I am an iOS developer and I have no problem handng over my 30% to Apple. The money I make in the app store with the 30% royalty is waaaaay more than I would make if I tried to sell my app on my own. Just ask all those millionaires out their that put their app in the Mac App store how well their apps are doing. A lot of people made more money in 2 weeks than they made in a year doing it themselves.
They already gave money to Apple to cover the free app when they signed up to be a developer. Beyond the app itself (which has already been paid for), it doesn't cost Apple a single dime to get Kindle books or Netflix movies to an iOS device.