Perhaps to discharge static in a plastic tank that's insulated with rubber mounts. Stop any sparks when you open the cap.
My guess is that in the event that the Foreman gets wrecked so violently and it tries to toss the gas tank, this cable is meant to keep the gas tank somewhat in place. Kind of like what you see in Nascar with the trunk lids and hoods secured by a cable. In the event the car gets wrecked, it keeps those parts with the car. Just my $.02 though!