Why do you lose reputation for down-voting?
In other words, why is StackOverflow's rating system biased (toward the positive side)? As as result, rating scores do not reflect reality.
Partly to dissuade people from gaming, it costs you to downvote someone else. Otherwise, there would be a flurry of downvotes from people hoping to gain an advantage for their answers based on the fact that their's appears both higher in the resulting display order and with positive (or at least zero) votes. Because it costs to you do the vote and it is locked in after a short window, it is less likely that people will use strategic downvoting.
If you don't feel that a post is bad enough for you to lose 1 rep over it to say it, then you're probably not convinced enough for your opinion to be truly worth displaying to the entire world. Or you have some rep whore issues. Why don't you just upvote answers that are correct? And if there aren't any, answer it yourself?
We like to think positive here. Welcome to meta. :D