It's happened a few times where I've forgotten to put the vehicle in park before turning off the engine. Luckily my Nissan actually turns off the engine, then the warnings go crazy telling you to put it in Park. If I ignore the warnings, at least the car won't drive away under its own power, although there is no guarantee it wouldn't roll if on an incline.
If FCA vehicles really don't shut off when you press the start/stop button, and you don't notice and also ignore all the warnings, then yeah, the car will roll away under power. Wouldn't the solution be for FCA to reprogram their start/stop software to shut off the engine when the button is pressed even when in any gear other than Park?