It's sort of incredible how people who are tech savvy have no business sense. I sometimes am annoyed at Apple, but what they are doing makes sense.This is the deal, as simply as I can put it.
With iPhone/iPad, Apple opened a new revenue stream for businesses - a new way to make money they were not able to before. They sell an app (say for $1) and Apple gets 30% of that sale. This is pretty standard. Now these companies can make millions they would not have made before. And of course Apple makes money too.
Then with certain apps, companies can sell other items inside the app, say books. So once again they have a new way to make more money, again money they were not getting before. With the in-app sales, Apple gets nothing. A company might sell a finite number of the apps, but the in-app sales is infinite. Apple comes along and says, hey, we should get a cut, as we were the ones who created the new revenue stream for you. And guess what, that's standard for any business!
It may sound like greed to some people, but that's good business. And it's common in all industries! Just as those companies selling in-app items can have expanding business, Apple business needs to expand too. If they did not take a cut, they would be silly, and losing out on huge growth opportunities. All app stores will do this. Andriod also does the 70-30 split. I'm not 100% sure about in-app purchases, but if not now, they certainly will. Business, people! Business!