It comes from Lemmy’s origin as a link aggregator. You can imagine that every link one the web has 0 votes by default. When you submit a post for the first time you are voting for it. You are saying “this is interesting to this community”. In this way the submitter is no different than any other voter, each upvote is saying “I think this is interesting” and every downvote is saying “I don’t think this is interesting”.