They are naturally imbalanced though (and they're slightly different for posts and comments) - in some sense they are for increasing and decreasing visibility, promoting and demoting i.e.

upvote being "I liked this so much I think more people should see it"

no-vote "I don't care about this, or dislike it - I don't think it should he seem by more people"

downvote then being "I hate this so much I wish to actively stop other people seeing it".

Perhaps an easier way of looking at it, is you go to see a gig, and the band finishes a song - if you like it, you applaud loudly. If you don't like it, you stay silent, or give a half-arsed polite applause. If you hate it, or find it offensive, you actively attempt to "booooooooo" them off the stage.

It may of course just be cultural differences about positivity and negativity.

In the other hand, it may of course just be me, being weird :)