Root.

Do you mean Admin account, rather than root? There's no need to be logged in as the root account.

 

I'm guessing it's because root is not a normal user in the sense that it's generally hidden--root is available to the Unix underbelly, but not something you'd normally ever see in the GUI. So this particular GUI element doesn't know what to do with it.

And as has already been said, you should never ever ever never be logged in as root unless you have a really good reason. I've been using OS X since the beginning, and I think I may have done this once (and I am a 'nix person). Don't do it - use sudo or the GUI equivalent if you need root access.