It's not a paradox, just a monopoly or de-facto that.

Platforms should be required to allow users to set an alternative default app store, much like browser etc..

Like any unregulated capitalism they will do everything to maximise profit. They get profit regardless of whether the original or the copy is being purchased. They get profit if a user gets scammed through in-app purchases.

Only because they are monopolies for the corresponding platforms. If customers could choose between smaller stores using different moderation strategies, the forces of competition would have found an optimal solution very fast.

I really don’t think Apple is consciously allowing these apps because they get profit either way. They’re very well aware that the long term success and profits of the App Store and the Apple ecosystem are based on users’ trust and frictionless experiences. In fact, Apple guidelines stressed pretty early on in the app store’s life (2010) that “we don’t need any more fart apps” in the App Store. I really don’t believe that they would risk their brand and image being affected by a very short-term focus on 30% commission on some scammy apps.

Also, more generally, even if this was conscious behaviour by Apple, I don’t think this really linked to “unregulated capitalism”. In any economic/political system you will find that some people serve their own interests while harming others or the public interest. Whether those “own interests” are amassing money, assets, power, access to goods/services or anything else deemed “valuable” in that system does not chance the dynamics of that basic conflict of interests which will cause trouble in any system. I’m afraid that’s human nature and there’s no system that can fully align those interests or avoid the negative effects of conflicts of interest.

They routinely feature games utilising dark psychological patterns for money extraction (e.g. Clash of Clans).