I lived in Japan eight years. I do agree that masking when one is sick is a prosocial thing to do. And that’s the way Japanese people traditionally did it: if you felt you were sick, you masked up (though it was far from 100% of people who did this). The thing is, if you were wearing a mask to the office (as I indeed saw people do) or just on the train, it was probably more prudent of you just to stay home.
Right. If, however, you know you are sick, you should probably not be out at all,.