The wheels are spinning because your automatic transmission is acting like a really shitty torque converter, transmission fluid is coupling all of the clutch plates together. I bet you could keep the wheels from spinning with your hand (please do not try though).
Undoubtedly doing 6k revs in neutral isn't great for your transmission, and running the car on cruise control while on jack-stands is just stupid. Any little mistake that would have been "damn, my car fell of the stands" will turn into "shitshitshitshit my car just took off through a wall and ran over someone."