Pedantic might be a little harsh, but I see your point.
You have no idea how spammy job board people (yes, I'm part of this crowd) can get when they find a new field to peddle their wares. We're just shameless. And if there's isn't some throttle/downvoting, these threads will become 90% "check out my job board" and 10% real jobs.
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.
You have no idea how spammy job board people (yes, I'm part of this crowd) can get when they find a new field to peddle their wares. We're just shameless. And if there's isn't some throttle/downvoting, these threads will become 90% "check out my job board" and 10% real jobs.