<p>Jeff initially proposed this in May 2011: <a href="http://meta.stackexchange.com/questions/90324/should-downvotes-on-questions-be-free">Should downvotes on questions be “free”?</a> </p>

<p>Since the community agreed, the proposal was implemented soon thereafter. The rationale behind it is that a question &amp; 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 <a href="http://blog.stackoverflow.com/2011/05/vote-for-this-question-or-the-kitten-gets-it/">get users to vote more</a>. </p>

<blockquote>
  <p>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!</p>
</blockquote>

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

<blockquote>
  <p>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&amp;A without questions, but having the wrong sorts of questions is far more dangerous. The fastest way to kill any Q&amp;A site is to flood it with low-quality questions.</p>
</blockquote>