If you noticed someones feet standing outside your dressing room door for more then a brief pause, wouldn't you be suspicious?
When you're in the room, you can't see anything outside. You see just a door with slanted slats...that's all you should see when you're standing outside of a properly installed door. When you're inside of a properly installed door, you can see the people outside.
And this isn't a mistake. Macy's admitted that their corporate policy encourages store owners to install the doors backwards on purpose so that store employees can "make rounds" through dressing rooms and see inside.
You're only going to see something if you stand right in front of it peeking through
While the angle on the slats will probably prevent them from seeing most people's faces (since someone outside can only see things below their natural eye line), it makes everything else completely visible to anyone nearby.
And you're all right, why do they even bother putting in doors?