Originally Posted by
Qualia
You are right. Now that I think about it, that's a strange small part of the story. Why couldn't the Keepers kill the Elementals on Azeroth when their spirits were originally bound there, but we can kill demons where their spirits are naturally bound to (the Twisting Nether) or the Elementals at the Elemental Plane where their spirits are bound artificially by the Keepers now? The way it works doesn't seem consistent. So, is it something special limited to Azeroth only due to the world-soul being an extremely powerful Titan, or is it something applied to the elementals everywhere else too? I guess we have to wait for an explanation from Blizzard, but given that this is just such a tiny bit of lore, we probably will never get one.