I don't think of it as a safety feature; more of a style or convenience feature that might have an additional safety benefit.
My former 2011 MKZ also had the 7-color ambient lights on the interior, but the ones on the door did not change to red when it was opened. If it were truly a safety feature primarily, I would expect it would be red every time you open the door; not just when the engine is running. For example, when I first enter the car, if the door is still open when I hit the ignition button, the lights on the door momentarily flash red but then turn whatever color I have currently selected for the ambient lighting. Weird. Why wouldn't it stay red as it does if I open a door once the engine is running?
My hunch is that the whole red thing came out of a customer clinic or feedback that something was needed for the open door. It's an evolutionary change in the ambient lighting system from the previous generation to the current one.