They are not anonymous in the blockchain. Take the URL for this article and change steemit.com in it to steemd.com and you can see the name of every person that voted, flagged, etc.
I do agree with your image though. People are referring to it as a down vote. It is not. It is a flag. Even other sites that have both upvote and downvote have a FLAG which means something different.
I understand that you can see who upvoted and who flagged, but the issue is that it is not mandatory to give a reason why a post or comment was flagged. While I would get my reputation destroyed if I went through and flagged all of a new users post I didn't like, if I had to say why I did each flag, users could see that my flags were not applicable to what was being flagged.
I actually called out the user who flagged my content about flagging (kind of funny) but he she could just not like me, disagree, have hit the button by accident, but I have no way to contact or dispute the flag.
I feel that needs to addressed and you are a user I respect @dwinblood so I would love to have you in the comments on the post.