When I ran a retail store, we charged a commission when we sold third-party services. Nobody thought this was strange or childish.
The problem is its hard to argue that Apple is really selling the other product.
There are lots of apps that are free and just act as gateways to web services that work by charging their members a monthly subscription (like this Skydrive app, the Dropbox app, the Netflix app, etc). It's totally absurd that Apple should be getting 30% of their repeating revenue because somebody used a conveniently-placed purchase link rather than going through some other, more obscure method.
In short - Apple is doing no work for that 30%. They're just using the fact that they can cut you off via the AppStore to make sure they get some of your business's recurring revenue. It's not even a small amount - it's a third. It's monopolistic behaviour because Apple know that iOS users are such an attractive market. Apple aren't serving any content for that fee, and these companies have their own card processing provisions. It means that if you're Netflix, if somebody taps the "subscribe" link on an iOS device, you only get 2/3 of the revenue you used to get. And that 30% cut will be repeated forever.
Web browser vendors don't ask for a cut of every transaction that goes through a web browser (and every subsequent transaction of a repeating payment). No desktop operating system does.
Mobile operating systems are just becoming all about money and vendors and carriers and everyone just trying to take it away from developers (who aren't making as much as many seem to think).