He doesn't need one because he wouldn't be at risk of developing the 'sickness'. Ben's people know what they are doing and are not at the mercy of the island. Remember when Juliette makes her journey to the island? Richard drugs her and they enter with what we assume is a submarine. We don't know what they do to her after she passes out or what means of transportation they take for the majority of the trip. Whatever their method is, they have managed to alleviate all potential side effects.

When Ben tells Jack that calling the freighter is a mistake and that everyone on the island will die, I'm sure he is talking about something they will do once there but he could also mean that if they were to just leave the island by traditional means they will not survive.