Why can't I rate a thread that I didn't participate in?

Say I have a question. I search and find that three other people asked that question and got three different responses. I find the one with the best response and vote it up. It helped me, but I never had to post.

*shrug*

Just makes sense to me, but I don't ever vote here at VBWorld cause it doesn't help any. I vote at PerlMonks.