For questions as why this would be allowed to happen or why there aren't people around, etc.
I'm only taking a guess but it probably goes back to the societal culture of Japan, the idea of not making a scene and the like.
On subway trains, if a girl was being touched without permission (in the past at least) then they couldn't cause a scene and there are even cases where wrong doing was done by another person in the house, but the other people in said house were worried about reprecussions and scared and such.
Even though it's the modern age and I'm sure this is less common now, I'm sure it still happens. (possibly, like I said it's speculation based on real things I've heard about).