Is this not clearly answered by a desire to maintain a cohesive user experience and even potentially security, like to not have people accidentally think they are using Apple Pay when they aren’t? Reminds me of network QoS controls being confused for favoritism.
The implicit assumption that Apple Pay is more secure is the problem here. If it is really about security or cohesive experience, then Apple should have a transparent process for approving "secure" payment methods for any vendors and allow the user to set any arbitrary default payment method in the cohesive style while putting other methods (especially Apply Pay for example in the non-cohesive style).
Not saying Apple is wrong or right, but your response isn’t considering the actual end-to-end user experience that will result from it, which is the part that Apple does care about.
But also, users who really care about end-to-end user experience will automatically prefer standard payment apps. So, then bad apps should automatically get weeded out.
Finally, it is possible I admit, that here, we have "What's good for an Apple user?" vs "What's good for long term goals of the system?". Because it is quite possible that the long-term cost that happens because of the monopolistic practices of Apple only impact poor users negatively, and not rich users. And therefore, the only argument that actually works is, "This is bad for the whole population in general in long term, because of the potential for abuse in future by a company."
In such a case of potential abuse, it is possible that the users would then look the government to help them out, possibly at huge cost to the government. So, it might make financial sense on the part of the government to prevent such a situation from arising in the first place. And then, the real solution to this problem would be to break up Apple the phone hardware company from Apple the iOS company from Apple the payments company.