I do not see any reason mentioned by Google for Doze mode to get activated only when the device is stationary.
Is there any specific advantage for doing this only when the device is stationary? What is the need to even waste resources checking for motion detector readings, when this can simply be achieved whenever the screen is off.
I am asking this because my phone is in my pocket for most of the time and because of that Doze mode will not get activated. I think there are many more people like me, so I don't understand why google chose these specific constraints for Doze mode.