Cellphones in Japan are usually way ahead of the rest. Text messaging, watching TV on the phone and online shopping are wildly popular in Japan. So then, just imagine you are sending top secret messages or looking at porn on your cell. It could happen. You probably wouldn’t want anyone else to know about it, and that’s why these are selling so well.
They are called “Mail Block” in Japan. Like the privacy filter for the monitors, sticking this piece of plastic on your cellphone screen will narrow the viewing area so that screen data is visible only to persons directly in front of the monitor. The plain one costs about $2.50. (uh… nsfw guys)