On long-distance trains in Europe - at least in Germany, the place I'm most familiar with - there is no provision for "sensing" that the train has stopped and allowing the doors to be opened or a GPS reading that notes that the train is in the station, as a poster noted was the case in the UK. As you noted, it's the conductor or the driver who flips a switch when it is safe for the doors to open. At least in Germany - and possibly in all the European Union - train doors can not be opened while the train is in motion (and the mechanics are functioning properly). Even when the train has stopped one often has to wait a few seconds until the switch is flipped. I've never seen a train in Germany on which the doors could either be released for opening by a passenger or could all be opened at once by the train crew.
Another poster thought the door-opening system described by the OP originated in Germany in the 1920s. All German long-distance and local trains (with the exception of luxury multiple-unit trains) until, I would guess 1970 or so, had doors that could be opened at any time, even while the train was in motion. I remember signs on the doors to the outside noting that they did indeed open onto the track. The only instance in early German railroad history I can think of - there may be more - where a door could be opened only by pressing a button after being released by the driver or, in this case, pushing a handle, and all closed when the driver flipped a switch, was on the Berlin S-Bahn cars that dated from the late 1920s and were in use until a few years ago - but I would have to check some of my S-Bahn books to find out whether this door-closing mechanism was installed in the trains when they were delivered.
Rapid-transit cars might have other provisions, but I've never seen any in Germany.
The reasons for all this have been noted -weather, energy savings,etc., but there was also the factor of mechanical simplicity.