What amazes me is where they get all their strength from. We've had visits by young women with slender white arms who have twisted the taps way beyond what I thought was physically possible. And when I confront them with it, they deny they have done anything out of the ordinary and accuse me of being paranoid.
As I said, the offenders are always people who live on their own, especially if they have just bought their own place. An undone tap means a dripping tap which means a higher water bill. But I'm sure that there is much more to it than that. After all, people who share or cohabit also have to pay water bills, but they tend to be much more relaxed about turning their taps off, sometimes even leaving them running to spare the next person in the bathroom the effort of turning them on again. Most considerate.
No, I'm convinced all this tap-turning is a manifestation of the insecurities of modern life: turn the taps off, click the light switch, lock the front door, go back in and check the taps again... worry, worry, worry.
Or maybe they just think the dripping of a tap in the dead of night will drive them crazy. They live alone so that they can have their own space, maybe a room with a view, but what they want most is a silent bathroom.