Ask HN: Who can moderate comments downward, and why do they do so?

I can. I think a lot of people can, probably you just have to have enough karma.

My question, which I can't ask as an "Ask HN" is that there seems to be some flag on my account that prevents me from doing an "Ask HN"...

The HN guidelines (https://news.ycombinator.com/newsguidelines.html) list two things about downvotes: don't complain about them, that rarely adds anything to a discussion. And don't bait others to downvote you. To quote one of your recent comments "Mod me down again, I suppose." I'd add a third: don't take them too seriously and sometimes people click a button by accident (happened to me on mobile once).

The downvote button starts appearing when a user has 500 karma points, the threshold changes over the years, it might be 200 or 1000 now.

Your account is 10 days old and I see one comment downvoted and another might have been in the past but somebody else upvoted in the meantime and I'd say you'll be fine. Good quality, adds value, welcome to Hackernews!.