On many networks /map is restricted entirely. Similar commands that can determine connectivity are also often restricted. This is so an attacker can not easily determine which server to attack to cause the most damage. I would guess the leaf servers are hidden so the attacker can not do as much damage if he dislikes a certain client server/someone on that server. That makes little sense though as the hub servers would seem more 'vital' in terms of keeping the network connected. You can only ask the administration of that network as to their reasoning.
Hope that helps
