It makes sense, though: Apple is all about making things easier for the user at the expense of third party developers. Users don't need to deal with app reviewers denying bugfixes because they used the demo credentials on the wrong app, they can just be mad at apps for being buggy. Banks all around the world are paying a secret Apple tax to let Apple users pay with iPay or whatever it's called.
I think the situation with Valve is different. Valve has competition and has had competition before; game launchers on Windows (and Linux, I suppose) are perfectly viable, it's just that none of their competition put any effort into making their game launchers as good as Valve's.
I think Apple should be able to charge whatever percentages they want, as long as you can install alternative app stores. If their customers accept the higher app prices in exchange for Apple's convenience features, the percentages are perfectly fair.