It's downvoted because some random user with >=500 karma didn't like it and clicked the down arrow. That's all downvoting means here. There are no ethics for voting here.
In effect, voting is simply a measure of whether random HN users were pleased or displeased by your comment.
And you're not allowed to mention it, either. If your perfectly reasonable and polite comment gets downvoted to invisibility by random, angry users who won't tolerate diversity of opinion, then suck it up, comrade, and learn to please the hivemind--no complaining!
Of course, certain very-high-karma users can get away with saying things that would be flagged and downvoted to death if said by anyone else.
And that's what HN wants voting to be. Welcome to HN. Enjoy your stay, if you can.