a) more often than not there is no passenger, just you driving (at least in most circumstances)
b) I think the passenger power lock also doesn't lock the drivers side door if it's open. But yeah, i see this as the only way to possibly lock yourself out: you leave keys in the car, exit, shut door, passenger leaves, hits door lock, closes door. Well, if it ever happens you've got that roadside assistance number you can call.... if it only weren't with the manual in the glovebox... locked in the car...
I think the goal of this is to prevent the most common type of locking keys in the car. Driver leaves keys in car, opens door, locks it, closes it.
I know you might have driven some american or japanese car before that allowed you to hit the door lock as you exit the vehicle and this just became routine for you. It was routine on my mom's old Caravan and she locked herself out at least a couple of times. Now, think of the VW door lock as having an entirely different function. The button is barely ever used, it's actually more of an indicator that your door is locked. If you want to leave the vehicle you don't need to hit the button to unlock, you just pull the door knob. The only time you will ever need to touch the button is if you're picking someone up. The doors will be locked, as they always are when the car is moving, and you will need ot hit the door lock button to unlock the passenger door. That's the only time you should ever need to use the button.