HN doesn’t let you un-upvote something either, but you need to un-upvote something on HN far less often than on Svbtle. On Svbtle, it is very easy to accidentally “kudos” a post, because all you have to do is move your mouse over the kudos circle, and the site doesn’t warn you that doing so will trigger a kudos. On HN, at least you have to click a button-looking thing to trigger the non-undoable action of upvoting.