Negative externalities is culturally dependent. The Japanese consider the sight of a person eating while walking to be a negative externality. Are we going to start complaining here on HN about people doing it? No, we recognize that those taboos vary across time and space.
The difference is some taboos are just based on tradition, and others are based on actual damage. Nobody is hurt by seeing someone walking and eating -- they can just not look at them and the problem is solved. You can't not just hear the noise people inconsiderate not to use headphones make.
In cultures where the sight of certain things is taboo, telling a local person that they have to direct their gaze elsewhere feels as much of a rude imposition as someone's phone audio reaching your ears. Please try to step beyond your own cultural upbringing and realize how arbitrary and mutable all these things are.
