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.
YouTube has negotiated a deal that allows this, as did Amazon Video and a few others. Only huge companies can do this, which further makes the argument that what Apple is doing is abusive.
This is not true. There is no rule that states you must offer the same price when signing up if your service is multi platform. The rules only state you can’t discourage In-App purchase. I use several services where I signed up on the website because it was substantially cheaper than in-app.
