I suspect a bad hack at blizzard servers: If you click on them, they take you as their target before adding yout stuff to the inventory and disappearing afterwards. That makes me think they used CMSG_GOSSIP_HELLO and add the item/despawn the npc there - there is no looting window.
If the client is really sending CMSG_GOSSIP_HELLO, I guess it has to be implemented unsing ScriptDev2
EDIT: Such creatures are used for quest 1200, the questitem has a chance to be looted from item 35792 and that item, which is "looted" from these NPCs is created by a spell. So most probably their Hello function looks like "m_creature->CastSpell(47096, looter); m_creature->despawn();"