You're making an assumption that all job boards are of equal worth to HN'ers, when in actuality there's a wide variety. I'm sure some job boards (for example Techcrunch's Crunchboard) would get upvoted, while other less relevant ones such as Monster, etc. would get downvoted.

To argue that HN'rs are incapable of distinguishing between job boards that they personally find useful from ones they don't seems a weak argument.