Because Apple is getting a cut zaps are competition by contrast.

IMO it’s just market power. Nostr doesn’t have it yet. Apple has a more complicated answer claiming a zap is purchasing digital content and it’s their policy to force digital content purchases through their marketplace, which doesn’t actually make sense because the content is posted and zapped after the fact so their isn’t really an exchange happening. I can purchase digital content through a web browser/client but not a Nostr browser/client? It’s just market power.