At least one of the helicopters at Bonnaroo is a Lifeflight helicopter from Vanderbilt. They keep them running 24/7 for medical emergencies, because Vanderbilt is the only Level 1 trauma center in the state. If there were a life-threatening emergency, trust me, you would want the helicopter running and able to evacuate immediately. It may be an inconvenience, but it could be the difference between life and death. They rotate the helicopters throughout the day and night, to keep different pilots on shift. It's been that way for years now.
Are you saying the helicopter is running, motor idling, blades moving all day and all night? Probabaly not "running" 24/7. It takes only minutes to get one of those birds in the air. a running helicopter is very dangerous. If Vandy does put one the extra choppers at the farm, it most likely sits there all day and night, until needed.