The bag is in waterproof (for a while, at least) paper, explaining to its users that it is waterproof and can be used for disposing. The reader has to use his or her own imagination to come up with any uses for things like that in a plane.
Interestingly enough, the bag will stay very clean until used, as it is closed on the top, you have to tear it open. If I may come with a couple of cultural interpretations of this, I would say that this probably finds its cause in the Japanese taste for cleanliness, rooted in their enthusiasm for taoism. Purity. Also, the now well-known Japanese industrial term "Just-in-time" springs to mind. You really have to prepare for using this bag, as you have to unfold it and tear it open before you can use it. My guess is that most users have it ready for use just in time, with very little time to spare.
Extra points for delicate design.
The bag was sent to me by Shinobu Yoshino. Give him a big hand!