Why do you compare this to post upvoting/downvoting?
This is a totally different thing. The purpose of witness voting is to elect witnesses.
But, if you really want to compare them, then probably this is the closest version:
Upvote (post) = Approve Witness.
Neutral (post) = Ignored Witness.
Downvote (post) = Disapproved Witness (removed vote).
You can vote for a maximum of 30 witnesses, but you can always remove your vote from any of them.
Let's see what are the witnesses:
Witnesses are creating, validating blocks, maintaining the stability and the security of the network, so all of the active witnesses are useful. Actually their work is necessary for the Steem blockchain.
Who can become a STEEM Witness?
"Anyone who is willing to contribute to the Blockchain may become a Witness, however they must be elected through votes made by the Steemit Community." "To essentially become a Steemit Witness, an individual needs to be approved and voted by the Community, thus that person needs to have a good-track record and must be liked or respected by others in order to be upvoted as a witness in the first place." - Source: Steemitguide : What is a Exactly is a STEEM Witness and why every User should Vote !.
As I wrote before, you can remove your vote from any of them at any time (disapprove/un-vote them) (for example if they become inactive or disabled), but downvoting any of them would be meaningless. Actually, it would mean shitting where you eat.
Would you like to shit where you eat? I think you would not.