I think that regardless of your car rolling backwards or "going" backwards under power, it should not be happening after the power button been depressed. It is a programming error in my opinion, nothing more. When the car is under a certain speed let's say of 2 mph and the power button turned off, the parking break should be applied as a default and be able to be released only if I push down on that P button.
How many cars do you know if that automatically apply the parking brake at power down?
(FWIW, my 06 Prius will put itself in Park (when stopped or at VERY low speeds) if you press power to shut down the car while in D. It does not and cannot engage the parking brake, which is via a foot pedal.
Leaf apparently does the same thing: http://www.mynissanleaf.com/viewtopic.php?f=27&t=8492
, although I don't recall the results of me trying it myself. My Leaf also cannot automatically apply the parking brake either since it also became a foot pedal version w/the '13+ model year. '11 and '12 Leafs had an electronically activated one, not a foot pedal.)
BTW, a quick Google search for fmvss shutting off vehicle while in gear
http://www.safetyresearch.net/blog/arti ... ance-probe
. The "Ready to Roll" section seems most relevant. (I'm very familiar w/the 06 Prius' smart key system behavior and that of the "Intelligent Key" (in Nissan parlance) on my Leaf and my mom's 07 Altima Hybrid.)