I'll be honest and say that I'm surprised that people keep downvoting it to levels that I associate with malicious trolls, rather than let it stay hidden at -5

It was already negative when I saw it, so I neither up- nor downvoted. I suspect the title catches more attention than the normal not-bad-but-not-helpful post, so even when it passed -5, people were drawn to look at it, and when they saw the content they downvoted without noticing the current score.

Actually, I wonder how many LW voters think of it as expressing an opinion (I want to support or dis-support this), and how many think of it as sending a signal (I want the author to change behavior). I suspect the former are more likely to add votes even if the score's already clear, and the latter to vote only if they think the current score is insufficient.