And the cable on the left here is what the included Apple MacBook power cable looks like, with the little earth wire sticking out to screw on http://rinare.com/wp-content/uploads/image_archive/110801_2....

I think it's just a legacy thing - historically, the only things that needed grounding were static appliances like fridges, washing machines and air conditioners. The manual in our fridge has this instruction for earthing http://mayoyo.tokyo/jIe.jpg

I need to revise my comment that "I don't think I've ever seen a three-prong outlet in Japan", because now that I have power sockets on my mind, I just saw a three-prong outlet at my gym. So I guess newer buildings have them.