I specifically mentioned Facebook because of how Facebook's news feed algorithm works. It's engagement based, and those algorithms are easy to saturate to specific types of content.
Reddit has a fundamentally different approach to front-page content and discovery, and moderation in general.
But even on Reddit, which is fairly permissive, they have hard boundaries which have little, if anything, to do with the "law".
You're arguing with hypotheticals even through real world examples exist.
Reddit has lots of porn and it's nowhere to be seen on the frontpage.
Allowing something doesn't mean you shouldn't classify it and filter it.