johnacek that's because the tag is restricted so that only administrators can create discussions.
I think it works something like: if normal users can post, show button that opens login for guests. If normal users can't post, show "cannot start" for both members and guests.
If you wanted to replace the link that opens login with a "cannot start" disabled link for guests all of the time, you would have to create an extension and use our javascript API to change how the button is rendered.
