Poor grammar and spelling lowers the signal-to-noise ratio. Upvotes and downvotes are quite specifically designed to raise the signal-to-noise ratio on the site. It doesn't have to reflect on the person as an individual.

Having said that, I don't downvote on grammar/spelling alone (as I mentioned elsewhere), and I do cut a little more slack when a person is not a native speaker (but not 100% slack - better grammar and spelling are always preferred, other things being equal).

Then that's just too bad, I guess. HN is an English speaking site. The ability to articulate your thoughts in the medium chosen for the discourse is cost of entry. Poor grammar is a quick indicator of content quality to me. In fact, it's just about the only thing I will downvote. Maybe it's because I was raised by an English professor.

Edited: Typo (of course)

Grammar is a component of quality discourse, though I agree the parent overweights it.