Here is my explanation:

    the system is simple: you CANT force the robot turning arond an obstacle on the wrong side  that will make him turning around without seeing the "exit"
It s IMPOSSIBLE:
   Look at the sketch: To do an obstacle where the robot could turn around, you have to put nothing where I drawed a green circle and the robot will imediatly go on.
Now it is turning on the good sense, and it s nothing for him to find the "exit"
    And if you close this opportunity to the robot?
See the second sketch!