Safety First
For most of my life in aircraft engineering (40 years now) the aircraft belongs to the maintenance crew until all servicing is complete and the Captain has signed in the log as accepting the aircraft. Its all a matter of safety precautions.
Try working on an aircraft during maintenance and you'd soon appreciate why we don't want systems being operated or surfaces moved while we're going about our business. When we must do these things ourselves as part of the task, we walk around the aircraft, clearing all work areas and warning all staff before applying hydraulic power or moving flaps or control surfaces. Boeing may write the books but they're aircraft builders, not Health and Safety consultants. While we understand the need for fast turn-arounds, there is no justification to reduce safety standards in order to knock a few seconds off here and there.
Its what HSE is all about.