Warwickshire Police have been given extra time to detain the two persons arrested. There must be more to this than meets the eye!
Might be just that they needed more time to find someone who knew enough about gyrocopters to ask the right questions.
And I believe that there is still some value in switching off the engine regardless of rotor momentum: at least the propulsion propeller will become harmless almost instantly and you will be able to talk to the intruder more easily.
According to the nth hand eye witness account above they were going
away from refuelling, when you would not expect the rotor to be at speed either, but I suppose that even in the early stage of pre-rotation it is going to hit hard.