I assume that you are talking about voting for content.
Short answer is: Because it's OK to selfvote.

It's unrelated to being a witness or not. Witnesses provides security and reliability to the network. When they vote they are doing that with their "user's hat on".
There used to be a switch that voted for your own content automatically after posting, but because early-vote penalty window it was removed as it was sub-optimal for curation rewards.
Reddit auto-ups your own content and it really makes sense, also for Hive:
If you don't think your content deserves an upvote, why are you wasting blockchain resources for it?
Post what you would like to see.
Vote for what you would like to be voted.
Well, it's always subjective, so other users can disagree with the reward and eventually all engaged parties will reach the consensus about their up and down votes.