The 30% is for paid apps and in app purchases of app content. If you use the Amazon app, for example. They don't take a cut.
I'm not on board with the 30% commission explanation. On the Amazon ios app (normal retail app, not video only), I can buy everything but videos. Wouldn't they be hesitant to share 30% of the other billion dollars of revenue they do via ios?
Apple will only charge you the 30% of products sold through IAP.
The caveat here is that if you sell digital goods through the app you don't any option other than using Apple's IAP.
also, in another answer:
It also doesn't include all digital goods. Only those digital goods whose services would be consumed within the app are accounted for the 30% free by Apple.