Uh ... let's see ... Guests, Newbie, and Jr. Member are set to (X) - Disallow. I have no idea what is different between "Disallow" and "Deny". Ugh. Off to Google because the SMF programmers have forgotten how to reasonably label an input.
Update: Got it. Deny is subtractive. It overrides an implied Allow. Disallow does not override Allow.
That does not explain why the section disappears.