A change to the default robots.txt files was made in Joomla 3.3 to allow Google to access templates and media files by default to improve SEO.
This change is not applied automatically on upgrades and Joomla users are recommended to review the changes in the robots.txt.dist file. This file is located in the root folder of your site after upgrading to 3.3 or later versions.
If you haven't made any changes to the original robots.txt file, you can simply replace it with the new file. If you have changed your robots.txt, make sure you go through the new file carefully and and implement these change into your own robots.txt file.