Why is autologon in Windows 7 a security risk

That's the security risk it is referring to - anyone who obtains physical access to the machine is able to log on and access any of your data. If your computer is in a safe place and that isn't an issue for you, there's no need to worry about it!

Aside from the obvious risk that someone can walk up to your computer and use it, there is another minor security risk. When you set autologon, your password is now on the computer. It is encrypted and theoretically safe, but there's always a chance that it can be discovered.