Well, the point is that a lot of people post their site on HN as "Show HN", and the "intended audience" is then (or should be) "power users", which should also mean that "power users" (few as they might be) are potentially interested to your site, but you make sure that only a subset of these power users can access your site (not necessarily in full, sometimes a simple text summing up what the thing is about would be enough to either - as I do if the page doesn't load - discard it or get me interested enough to turn javascript on/use the "other" browser).

If you don't post your site on "Show HN" the missing "no-javascript message" is only a (small) lack of courtesy, if you do post it, it sounds (to me) like a plain lack of attention.

In the real world, you open a new shop in town, and advertise on the local newspaper "everyone is invited for the shop opening tuesday at 7:00 P.M.", than you put at the door a couple big guys that only let in those dressed "black tie". I do have a suitable attire, but I am not going home to change into it, not without having at least the possibility to peek inside your shop and see what it is about. Will I become a customer? Maybe yes, maybe not, but more likely not.