There's a tendency in discourse to make generalizations across large groups of people, which degrades our ability to see the nuance and diversity of opinion and perspective that clearly exists. This gets particularly pernicious when discussing contentious topics. Given the how damaging this can be (current political discourse in the US or here on HN is a good example), making such generalizations really needs to be justified.

In 'lcw's comment, the generalization doesn't add anything substantive to the comment. Indeed, one could read it as disparaging to the HN community because it's too naïve to understand the risks of LSD, rather than granting the members the benefit of the doubt that they're capable of making reasonable decisions. And that's ultimately the point: by making generalizations like this, it doesn't add anything substantive or constructive to the comment and ignores the wide range of opinions across the HN community.

Edit to add: As an aside, you mention that 'lcw's observation matches your perception. It's well known that our perceptions are often wrong, or at least skewed. That's why backing them up with data is important, to make sure our perceptions are consistent with reality.