I think this letter from today's Guardian sums up what I had in mind when I said it was essentially a question of good manners:

I regularly used to smile and say hello to a woman at my son's school who wears a niqab, but I gave up when I realised I had no idea if I was getting anything back - a smile, an acknowledgement of my greeting. As one of the 9 million people in the UK with a hearing loss, I wouldn't have heard her if she'd said anything. I need to see someone's face to have a conversation with them. I would like to make friends, but it's been made impossible for me.

Fiona Pickett
London