Stop
While I do agree this is a dangerous practise when there are passengers
I did, for a while fly a DC-8 freighter and the DC-8 doesn’t have an APU. If you had to divert to an airport that did not have an air starter, then one engine would be left running. This would allow a cross bleed start to be performed once refuelling had finished.
If you shut down the engine in error then another freighter would have to fly an air starter unit out to you before you could be on your way.
If the a/c in question (and you don’t specify) is a 737 then it would be a no no in my book as you have the fuel receptacle by the number 2 engine and pax passing the number 1 engine while disembarked and boarding.
If it was a 727 and the engines are up and out the way then it’s a different story completely. Boils down to safety.
If in doubt don’t is always a good motto.