This question already has an answer here:
- Why do you lose reputation for down-voting? 18 answers
I never understood this asymmetry. You can up-vote all you want (well, up to some daily limit), but down-votes are discouraged by decreasing your rep.
If we're worried about down-vote abuse, then a daily limit would fix that most likely.
But why punish someone for rating a poorly written question as such? With this policy people are more likely to vote to close a question (it's free).