Anti-spam measures exist in SMTP, but the broader moral imperative does not.
I can disable my spam filter, and no one will know or care except for me (if I get a lot of spam). On the other hand, it is generally considered immoral to allow people to disable anti-Bad Person filters, because then two Bad People could use the network to communicate. That creates a lot of potential for drama, large public arguments, and network fracturing that isn't present in legacy systems.
People who use the service to plan murders?
People who disagree with your politics?
People who (accurately) quote a book written 200 years ago that uses a word you don't like?
People whose skin color you don't like?
Even the "ban what is illegal by law", aside from some common stuff, varies from country to country. Anything above that is pretty much politics.
Like, it's fair to say "okay, this community is to discuss X, we don't do politics coz that's always divisive", but the moment mods try to play moral guide and decide what's wrong think and what isn't any bias they have will be magnified by the power they have.
I can disable my spam filter, and no one will know or care except for me (if I get a lot of spam). On the other hand, it is generally considered immoral to allow people to disable anti-Bad Person filters, because then two Bad People could use the network to communicate. That creates a lot of potential for drama, large public arguments, and network fracturing that isn't present in legacy systems.