Blocks won't show for anonymous (logged-out) users...

I have set up a block to appear inside this group: http://archstl.org/education — it displays fine for any member of that group who is logged in. But it will not appear if you are logged out, or an anonymous site visitor.

I checked the block settings, and there are no settings for OG Block Visibility besides the 'which groups should this block appear in' setting...

In og_block_visibility.module, line 207, I found the following code:

function og_block_visibility_check($module, $delta) {
  // Are we in a group?
  if ($group = og_get_group_context()) {
    // Does the current group have this block assigned to it?
    $groups = og_block_visibility_get_assignments($module, $delta);    
    if (in_array($group->nid, $groups)) {
      // Is the block public?
      if ($group->og_selective <= OG_MODERATED) {
        return TRUE;
      }
      // Is current user a member of this group?
      global $user;
      if (in_array($group->nid, array_keys($user->og_groups))) {
        return TRUE;
      }
    }
  }
}

It seems the 'Is the block public?' access check is getting its value from somewhere, but I can't find a way to make a block 'public' or not... and I couldn't find any permissions on the permissions admin page to expose this control on the blocks admin page. Is this simply a missing setting?