"self-evidently full of excrement their droppings" seemed both scornful and snarky to me.
> a lot of postings here (or excerpts from them) are plainly BS
In principle I agree because that's what human beings do, but I don't agree with your word "plainly". That implies that there exists an objective perspective that nearly everyone can easily agree on, as long as they're in good faith. But there's no such perspective—not on divisive topics. The situation is far from that.
> But is so wrong or uncivil about pointing out this simple fact
What counts as BS vs. not-BS, or true vs. false, is no "simple fact"—it's the core of the hardest problem. If you treat it as a simple fact (for example, "I'll just point out the simple fact that what you said is BS"), you're likely to fall into flamebait without intending to.
It's better to explain patiently and respectfully why you believe someone is wrong. That's what we want on HN: to correct false information with true information, and bad arguments with better arguments—always respectfully and assuming good faith.
> Whether the poster is simply naive, or has less charitable motivations
If there's one thing I've learned it's that internet commenters are much too quick to make assumptions about others' motivations. The trouble is that these assumptions don't feel like assumptions; they just feel like completely-obvious-observations. Of course the other side has their own completely-obvious-observations. This is how we end up with flamewars.
The solution is to refrain from making claims about other people's motivations, which you can't know because you don't have direct access to them, and which you're likely to be wrong about because we have so little information about each other from the tiny blobs of text that we're exchanging. Also, humans really dislike having their motivations commented on by others—so even if you've assessed them correctly, it's one of the worst tacks you can take. As a bonus, not doing so will make your comments more likely to be substantive and less likely to break the site guidelines.
By the way, I think I noticed a pattern in the following phrases in your comment: "entirely reasonable", "easily verified", "plainly", "simply", "simple fact", "there's just no ifs, buts, or mitigating factors"; as well as "self evident" from your comment above.
To me these phrases suggest a belief that the material under discussion is somehow obvious or transparent, and that it's obvious what's true and what's false (at least on certain points—I get that you aren't saying this about everything).
When you have that feeling and then you encounter someone who's arguing against that simple/obvious/plain truth, you're almost forced to conclude something about the other person: they must be naive, disingenuous, obtuse—something like that—or maybe they're just dumb or bad. Why else would they deny the obvious?
What I'd love to persuade you of is that this is a bad position to be commenting from on the internet (where we know so little about each other and don't have pre-existing relationships). It's likely to make your comments seem dismissive and/or aggressive and/or personal, even as it will feel completely obvious to you that you're "simply" pointing out facts. That's again how we get flamewars.
Instead, we all need to realize that the world is a much, much, much bigger place than we thought. There are lots of people out there who have opposite views and feelings to our own, and who therefore have a different idea of what is obvious and transparent. This is not because they're disingenuous, obtuse, or dumb. It's because they have a different background than we do, and a different working set of information.
Evolutionarily we're hard-wired towards assuming that other people feel and look and think the same as we do. In the mythical primordial jungle, we didn't encounter many others; and if we did, we probably tried to kill them (and they us).
Now suddenly on the internet we're exposed, not just to a few more others, but to all the others: millions of others with totally different backgrounds, information, feelings. When we encounter them, our immediate reflex is the primordial fear of kill-or-be-killed—that's how hard-wiring works. And what is flamewar but the internet version of kill-or-be-killed?
What we need to do instead is learn: learn that our existing assumptions, beliefs, feelings are too small for this new situation; learn to pause our first responses rather than reflexively expressing them; learn to look for information that might not be obvious/simple/plain, and to be willing to receive such information from the other.
To me this is what Hacker News is about: an experiment in learning from each other, which first means tolerating each other: staying connected long enough to get over the initial response of "what kind of idiot, asshole, or sociopath is this" and move into an openness to "what could be the reasons why this person has such a different sense of what's obvious than I do?"
Tolerance is one of the vanilla virtues we all think we're practicing, but actually we've barely begun to realize what it is. Tolerance feels terrible. It means bearing the pressure that the other person is putting on you—just as a beam engineered to a certain tolerance is able to carry so much load without snapping. In human terms, tolerance means being willing to bear the "unpleasant manifestations of others" right up to your snapping point and then letting it stretch your snapping point. Not only is that not easy, it's one of the hardest things a person can do.
I ended up writing a mini-essay here. That sometimes happens. I hope it didn't stop being relevant to you!
(Edit: you put in the last part of your comment while I was writing the above, so I'll respond with an edit too. No, it's not ok to replace that language with words like "nonsense" or "balderdash". Those still count as name-calling in the sense that https://news.ycombinator.com/newsguidelines.html uses the term; and they don't solve the real problem. Since my reply below is an attempt to explain the real problem, I won't go further here.)