Yes, it is. To the Japanese, it seems like using the toilet in public! They are totally disgusted by foreigners who expel body fluids publicly. When I taught in Japan, students who had bad colds would swallow the excess mucus in order to avoid having it come out of them. That is also why you will see many people in Japan wearing face masks when they have even the slightest sign of a cold. There also seems to be a moral responsibility that people take on when they get a cold to make sure that absolutely no one else catches their cold.