I suggest everyone that can (if you are willing to create a user) to request a review saying that the "Red" rating is wrong, And also comment that the site is "No safety issues" at:
http://www.siteadvisor.com/sites/www.askubuntu.com
It seems that the site might be ran by user submissions, possibly one user filed a false report - maybe the only report...
But If that is the case It seems they have not reviewed the report properly, and certainly have not documented the reasons for the "Red" rating.
So If you use the site and know it is safe submit a report.