And that's exactly the problem. Can you imagine if Microsoft started enforcing a rule where anything you bought using your Windows PC gave them a 30% cut? If you were to sign up for an Apple Music subscription through iTunes on Windows and were forced to use a special "MS in app payment system." And even if you're listening to it primarily on your phone, or Sonos, or any other platform, Microsoft still gets a 30% cut every single month? It's ridiculous. And that's exactly what Apple is doing here.
I'm fine with them taking a small processing fee if it's handled through their payment system. That makes sense. But 30%? No way.
Let me break it down for you.
iPhone -> App Store -> App
Windows -> app store not needed -> App
Mac -> app store not needed -> App