This isn't true, it's just that the spatial constraints IRL mean there's never been a need to scale moderation of town squares beyond intuitive methods. Go to your physical town square and start screaming slurs and threats of violence at passers-by, and see how long it takes before the community "censors" you.

In developing and developed countries I've had (ostensibly) mentally ill people shouting epithets at passerbys and they're just ignored. So I think this is more about cultural norms.

Moreover, the case you mention is simple. If a spammer joins a forum and starts spamming, there's usually broad support to kick them out. Once the argument becomes ideological, that's when sentiment is a lot more mixed