Apple operates globally and wants to avoid legal issues, it takes a conservative approach, choosing to ban adult content outright rather than risk hosting material that could be deemed obscene in certain jurisdictions (U.S. obscenity laws). However, this is a corporate decision, not a legal obligation, since mainstream pornography is protected under U.S. free speech laws.
The key issue in this discussion is that Apple’s ban is a form of corporate censorship, not government enforcement. They are within their rights to curate their platform, but this decision limits individual choice, despite the fact that users can still access the same content through browsers or alternative app stores. So while obscenity laws set legal boundaries, Apple’s stance goes beyond them by enforcing a private, moral-based restriction rather than a strictly legal one.