Where does it say that it’s assur? I would assume its accepted on so
E places and not in others. Where I grew up if you didn’t wish a passerbyer good shabbos you were extremely rude, but I’m sure on the streets of bp and Lakewood the opposite is true. I would think though that if a woman was walking and said good shabbos, and you put your head down and kept walking that that would do more damage to the image of frumkeit you are trying to display than not.