In Destiny's lore, the Cabal are use to a much higher atmospheric pressure. Earth's atmospheric pressure is so low that when the cabal lose their helmets their black blood evaporates. An instant death. You can see this in Destiny by killing cabal with headshots. However, the gameplay reveal showed Ghaul with his helmet off when the he faced the Guardian. Unless Bungie decided to change the lore, Ghaul isn't an ordinary Cabal. Bungie did say Ghaul wants the Traveler for himself. What do you think?