Apple's 30% only applies to digital content that further enriches the core app experience. That's my understanding. For instance, Apple does not take 30% of all Amazon purchases through the Amazon iOS app either. Nor do they skim 30% from any retail app.
However, if your app happens to be a game (a "free" game, for instance) and your in-app purchases enhance that game in some way (e.g. unlocking features) then Apple gets a cut of that.
However, if your app happens to be a game (a "free" game, for instance) and your in-app purchases enhance that game in some way (e.g. unlocking features) then Apple gets a cut of that.