It's a Google policy called "First Click Free":

To implement First Click Free, you need to allow all users who find a document on your site via Google search to see the full text of that document, even if they have not registered or subscribed to see that content. The user's first click to your content area is free. However, once that user clicks a link on the original page, you can require them to sign in or register to read further.

http://support.google.com/webmasters/bin/answer.py?hl=en&...