The thing is, you're not paying 30%/15% extra - if I recall correctly, that is against the terms of the App Store. Developers/companies can't charge a markup to make up for the bit Apple skims off the top - so instead of, say, Spotify getting all of my $10 from an IAP Premium subscription, they get $7. If I sign up for their premium service on their website, however, they get all $10.