This a bit silly since there are no in-app purchases to circumvent. It's more of Apple forbidding you to say in app "oh yeah, I'm also selling stuff, not here though". I think such restriction should be illegal clause because of customer or competition laws or sth. They aren't since Apple has them, but should be. Regulators should descend on platform owners and force them to share. Like they are forcing telecoms to sell services in bulk to virtual operators.
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.