But also, people are just humongously inattentive. There's no reason for all this. Each stall has a way to identify as occupied.

#1: you can see the occupant's feet without moving your head. Cast your eyes down slightly.

#2 & #3: if the door is locked, the door rests slightly further in than when it's not, so there's more of a visible black border from the dividing wall support.

#4: The end stall's door only closes if it's locked; it's the only door that swings inward, not outward.