Apple has no more business taking a 30% cut just because the content is on iOS than Microsoft has taking a 30% cut of items I purchase online just because I used a Windows machine to do so. My TV manufacturer Samsung can not demand they get a 30% cut of any pay-per-view movies I order since I did it via the TV I paid for.
This cut comes from software/data* purchased in the iTunes store. Your examples are equivalent to Apple taking a cut if you call a retailer and make a purchase using an iPhone. If Microsoft operates a store, internet or brick and mortar, then they would certainly take a cut of any software or data they sell.
*by data I really mean copyrighted material like music and movies.