The legal reasoning behind this is very, very simple and has nothing to do with beans.
Propellors are considered to be part of the engine. Run your engine all day long if you like, but don't put any time in your logbook.
Rotor blades are airframe parts ("rotary wings"). When the blades are turning, the airframe is moving. When the airframe is moving, you are logging time. Period. Why there needs to be any discussion about this is astounding.