As I see it, ground operation of a helicopter without a pilot at the controls is explicitly authorized by the FAA via AC 91-32, Safety in and Around Helicopters:
"Quick Turnaround. Helicopter pilots may use a quick turnaround operation to avoid delays at airport terminals and minimize stop/start cycles of the engine. During the quick turnaround procedure, pilots sometimes leave the cockpit while the engine and rotors are turning. If possible, the pilot should remain at the flight controls whenever the engine is running and rotors are turning; however, if it is necessary for the pilot to leave the controls of a running machine, the pilot should observe the following safety precautions:
(1) Ensure that wind conditions will allow such an operation to be conducted safely.
(2) Ensure that all controls are secured in accordance with the aircraft flight manual and the company operations manual.
(3) Reduce rotor and/or engine rpm to ground idle or minimum recommended settings.
(4) Ensure that all passengers are closely supervised by appropriately trained crew members.
(5) Ensure that no unauthorized person(s) approach the aircraft unless properly escorted."