As a counterpoint, if Apple relaxed this rule, then app store revenue would plummet. Everyone would make their app free and virtually non-functional, then sell subscriptions via in-app links that direct the purchase to their own fulfillment service. Nobody's entitled to a business model, but I think it's useful to take this devil's advocate view.

Regulators know exactly what Apple is doing here, and have so far declined to take action. That's an explicit decision, not an oversight.