This what Apple tells you. But the real and only reason for this is money. It is all a part of Apples wallet garden business model. It is simply not true that you can not implement secure payment if you allow access to NFC.
But take this:
Apple could have allowed a system, where you can select a payment provider (piece of software that follows an interface description). The new payment provider could use a different clearing method, that differs from ApplePay.
So there are tons of possible solutions, to open the NFC interface to developers.
Another one - a developer could use an Apple certificate if he wants to access the interface (like it is done with e.g. Push Notifications today).
Apple is only blocking access cause it has the role of the gatekeeper and it doesn't want to allow competition on its platform. Apple iOS strategy is "
Lock competition out, lock customers in" - this is also documented in Apple internal emails:
https://www.theverge.com/c/22611236...-liberty-app-store-schiller-sweeney-cook-jobs