Leaving the controls with rotors turning has a time and place... I have done it myself thousands of times without incident, safely and legally.
3 minutes to load passengers, 6 minute flight, 3 minutes to unload passengers and secure cabin, 3 minute return flight. Booked in 15 minute blocks all day. Starting up and shutting down every time not possible with that aircraft within the 15 minute window. For most the flights are already "too expensive". Passengers briefed beforehand including not approaching the running aircraft without the pilot and to stay seated until instructed.
Despite the briefing and pre landing brief I had a guy ignoring both briefings, hop out and make a beeline back towards the tail... I suspect shutting down wouldn't have made much difference in that case.. perhaps the extra delay of shutting down rather than control locks and frictions might have prevented me from intercepting him?
I have seen people take the piss too... years ago I watched a guy land an R22 in a campground, walk into the hotel and chat for 20 minutes or so with the 22 idling away... plenty of people / kids / dogs wandering around the campground. It's illegal to leave the controls of a robinson here with or without locks as its prohibited in the POH. Stupid and totally unnecessary.