They don't need to be on the ground to release as a rule, there are just situations which make it very difficult or impossible for them to release without being on the ground. The bald eagle courtship would be suicide if they could not release while in the air. For light loads they are quite good at releasing stuff in the air and will pass nesting materials and food to each other while in flight.
https://youtu.be/gGjPrX84SMU