The AS350/355 series collective lock should not be "eas(il)y knocked off." The lock plate should be held firmly in place either on the grip area or stud by by tension of the collective bottoming. It should require definite downward pressure on the collective to engage/disengage the lock plate. Cyclic friction is another question altogether, difficult ergonomically, unlike the collective lock.
Servos will 'motor', moving controls spontaneously, which is why people think turning the hydraulics off is a good idea. In some aircraft the hydraulics off trim point is not the same you set your controls before you exit, turning off the boost allows them to return to the designed neutral setting- the collective will rise to an approximate mid-point on the Astar which, at flight idle, is a significant amount of lift. I wouldn't trust force trim to maintain control positions either: if I couldn't friction/clamp/lock all the controls, including throttle, I wouldn't consider leaving the seat.
Externals have to be considered as well. Is the aircraft securely placed? Wind and weather not a problem? And the groundlings- is it better to manage the area personally or is it better to shut down? Not being able to restart after the ground operations can be a safety issue if you're not at a regularly secured landing facility.
It always puzzles me that the Europeans I know are so very well trained but hobbled by excessive regulation at home. NO- our alleged 'cowboy' attitude isn't the reason various Yank pilots buy the farm- if the very real possibility that being stupid doesn't stop a pilot, a rule won't. If that is the problem here, it's a trainig\ng issue from Day One of the training process- my opinion.