The reasoning behind the auto-locking feature is that when the doors are locked it adds to rigidity of the car and thus decreases the likelyhood of the passanger cabin collapsing on the occupants. Auto unlocking the doors would completely defeat the reason for that feature.

The actual mechanism of how the door works as kind of "configurable deformation zone" usually involves somewhat thick steel rod running down the middle of the door that on hinge side abbuts similar strength member in the chasis and on the latch side connects to the latch. The latch has two distinct positions depending on whether the door is just latched or locked and the only latched position is not strong enough to hold the potential impact forces..