I don't think we fully understasnd what they're trying to accomplish.
It seems to me that they want the fan to operate automatically. This means two things:
First, the fan will run for a period after the bather has left the room; and,
Second, the designer believes that the room won't be dry until the fan is drawing in dry air. Hence, placing the sensor closer to the fan.
Sure, most folks would simply use a timer. Trouble is, a timer measures time, not moisture. Plus, there is something creepy about flipping the switch 'off' and having the fan continue running.
Now, if only the fan had a 'foul odor' sensor!
