The problem isn’t apps that will use other reputable payment processors, the problem is that when Apple has no control over what the app developer claims is a payment processor the app developer can build their own fake one which is there just to build a database of credit card information to sell to criminals.
And even if it isn’t a scam it can still be a poorly written payment solution sending information unencrypted, something that is now on the consumer to vet before typing in their information.
People should be warned that they need to be vigilant and do their own research to make sure that where they are typing their information is legit and safe. I am all for the free choice so of course I think the developers should be allowed to do it, but I also think the consumer has a right to know that Apple is unable to vet it so anything could happen with the information you type there.