Jeff initially proposed this in May 2011: [Should downvotes on questions be “free”?](http://meta.stackexchange.com/questions/90324/should-downvotes-on-questions-be-free) 

Since the community agreed, the proposal was implemented soon thereafter. The rationale behind it is that a question & answer site needs good questions to survive—or at least be more efficient at telling the good from the bad questions—and that removing the cost of downvoting questions would [get users to vote more][1]. 

> Perhaps we should institute a new policy: every time you forget to vote a great question up, or a bad question down — a kitten gets it!

The follow-up blog post is here. It talks about [optimizing For pearls, not sand](http://blog.stackoverflow.com/2011/06/optimizing-for-pearls-not-sand/) and the value of good questions, which basically comes down to incentivizing (I hate that word) you to downvote the bad stuff.

> That’s why we’re determined to keep question quality high, even at the cost of refusing a little sand. It’s true that you can’t have Q&A without questions, but having the wrong sorts of questions is far more dangerous. The fastest way to kill any Q&A site is to flood it with low-quality questions.

  [1]: http://blog.stackoverflow.com/2011/05/vote-for-this-question-or-the-kitten-gets-it/