Submissions can be upvoted, or they can be flagged. They can't be downvoted. No one confused the upvote button for the flag button.
Comments can be upvoted, or downvoted, or flagged. People sometimes mistake the up and downvote buttons.
User X posts a news that gets 2 points in a few minutes. With point 3 the news would stay a few seconds in the home page, enough if it is interesting for other users to be upvoted and end stable in the home page.
This doesn't happen. People can't downvote submissions. Comments don't get ranked like that on /newcomments.
You are right, flagging is reversible, so what I said only applies to comments. Still I can't see good reasons for not making users able to undo their votes.
Comments
HN has submissions and comments.
Submissions can be upvoted, or they can be flagged. They can't be downvoted. No one confused the upvote button for the flag button.
Comments can be upvoted, or downvoted, or flagged. People sometimes mistake the up and downvote buttons.
This doesn't happen. People can't downvote submissions. Comments don't get ranked like that on /newcomments.
You are right, flagging is reversible, so what I said only applies to comments. Still I can't see good reasons for not making users able to undo their votes.