I don't understand why yet. Why is it important to you to know whether someone is listening to music or not? Why do you need to be able to tell at all?
it's just fucking with social interaction; maybe that's the point
totally understandable as the "right to be left alone" or privacy is essential but undeniably curtailed when in public.
i hope you can appreciate the collective imperative here and acknowledge that it would be super fucked up if everyone would be wearing headphones in public.