One way of you to have responded without leveling false accusations would have been to say something like, "let's not forget the other side, upvoting." Just because I didn't mention it doesn't mean I wouldn't have been receptive to it. I would have been very receptive, I am very receptive. But instead of putting it that way, you decided to take an oppositional posture which presupposes, falsely, that I would have been resistant to the point. You manufactured a conflict where there had been none.

To answer your question:

But your suggested fix - penalizing a downvoter two whole karma points - attaches no corresponding penalty to upvoting. Why not?

let me rephrase it:

Why didn't you think of everything?

I think the answer is obvious.

So, let's turn to upvoting. One solution for upvoting is to end it. Make it just downvoting. If the purpose of moderation is to protect the community from trolls, then all you need is the downvote.