There are acceptable reasons for a pilot to leave a machine at ground idle, as has been posted - in the bush, at high altitude, etc. - where it is considered to be less of a risk than shutting down and take a chance on re-starting, or safer to board or discharge passengers/freight, etc.
However, the OP tells us that this occurred at an airfield in the UK. Somewhere where it is illegal under the ANO to leave a helicopter to fend for itself under power - and I would have thought totally un-necessary given that there ought to be ground-staff to operate fuel pumps, etc.
I knew when I saw the original post that there would be the inevitable 'it's wrong - no, it's right' argument about a pilot leaving the helicopter with the rotors under power, or immediately after shutdown - but in this instance I think it was totally wrong, and un-acceptable at an airfield.
As for the foot-out-of-the-door and the chop-throttle-and-depart debate, it's an attitude thing. Some pilots display and practice a more professional attitude than others.
I know which I prefer in my pilots.