I'm sure they can't be there to 'save water', as they auto flush as soon as you stand up, knowing good and well you still gotta wipe your ass and flush again anyways...
Auto flushing is also not consistent -- it really depends on the toilet. I've seen some that are very good, doesn't flush until you get up, and others that are so bad they flush while you're sitting on it.
I feel like they should tie the auto flush sensor to the door hinge, since any movement there indicates no ones actively sitting on the toilet. Unless you're one of those people who refuse to the lock the door...