It's the myth of "Community Standards" or, to be more precise "Contemporary Community Standards", which is a concept in American law and, I'm sure, the laws of other countries, which states that:
> Jurors are the judges of contemporary community standards, based upon their knowledge of the norms of the community from which they may come. The juror must also decide whether the "average person" in applying such standards would find that the disputed material appeals to "prurient interest" or is "patently offensive." Experts testimony may be used to testify about the nature of the contemporary community standards,' but such testimony is not constitutionally required.
https://definitions.uslegal.com/c/contemporary-community-sta...
Back in the Dark Twentieth, you could maintain the polite fiction that even broadcast media was bound by these standards, as the FCC would go after local affiliate stations, not the mothership, for violating broadcast regulations. In theory, and, to some extent, in practice, local stations could regulate what got shown, so as to prevent what you mention: Distant townies trying to impose their standards on the locals.
This breaks down in the Internet Era, of course, because, while a website may claim to have Community Standards, a website is not a community. A website cannot have Community Standards, Contemporary or otherwise, because the people it has contributing to it are a pseudo-random mix of some vaguely-defined demographic, and, as you yourself show, can and will differ sharply on precisely the kinds of things Community Standards presume a strong majority in a community can agree on.
The Liberalization of the world has ripped a lot of veils off the cultural standards we used to abide by, and turned polite fictions, the kinds of things all the adults in the room could admit privately were not laws of nature but laws of local social norms, into, at long last, simple fictions, as might be found in a storybook. "Community Standards" is one such polite fiction, and it's been replaced by the standards of the platform owners.
We can remove the platform owners by federating and decentralizing, but "Community Standards", as-was, isn't coming back. Communication is far too important to allow the previous geographic segregation to reassert itself.