Sisloesid-
"At rest" is stationary, no longer moving. Landing is the process of putting a flying aircraft on the ground, as in "landing aircraft have the right of way". An aircraft that has landed might taxi to it's resting position. A helicopter can be at rest with turning rotors.
You might not be able to think of a good reason to leave the pilot's seat with the rotors turning, that doesn't mean that it is unreasonable to do so. Accidents that occur after the pilot left the pilot seat don't prove that the practice is unsafe anymore than accidents that happen with a pilot at the controls prove that unsafe. There are ways of reducing risk in both circumstances, and as far as I know this pilot was blameless, it might have happened with all the best practices complete and in place.
Boudreaux Bob
I don't fly a B3.