The problem is that the people who talk about this have no basis for the claim. It feels like it's happening, so they say it is. Other people also say it is, so there is social proof. Anyone who disagrees is reframed as a shill, puppet, useful idiot and the like, leaving no feeling of uncertainty internally.

As far as I can tell this dynamic has nothing to do with reality. Reality is that the HN community has millions of people, is diverse in many ways including internationally, and is divided on divisive topics. That is already enough to explain the comments that show up. But none of us is wired for dealing with that. We're wired for loyalty to our tribe, needing to feel safe, and suspecting outsiders.

It's painful to encounter a sharp opposing view, and people are angrily sharp on divisive topics. Reframing the other as a foreign spy or whatever insulates you from that. It relieves you from considering what truth there might be in that view, and reinforces loyalty to your own—at the cost of feeling surrounded by infiltrators and enemies. This is poisonous to thoughtful discussion, which depends on people being willing to open to differences and truths they may not yet see.

Since the actual phenomenon is vanishingly rare compared to the insinuations people make about it, we have a rule that users not post such insinuations without evidence. A feeling is not evidence. Even the sense "there are a lot of green accounts saying things I disagree with" is usually just a feeling, because we notice things we dislike more vividly than we notice anything else [1]. People on the opposite side have the opposite perceptions, but the identical feelings.

Notice how when people post claims about astroturfing, trolls, and spies on HN, they don't include links; just as when they make claims about "threads about $topic", they don't include links. Why is this? If the perception were of reality rather than a feeling, specific examples would always be available, yet in practice they almost never are. The discussion falls apart when specific cases are mentioned because (a) people never agree about those, and (b) such data as there is never supports the claim. The discussion always stays in a mist of generality, breeding bacteria of suspicion.

[1] Does anybody know or have a name for this bias? It's a huge factor in these discussions and it needs a good name.