Nuns are not confined to the church, they can and have gone out into public many times, for various reasons.
Also do you mean to say this in a way that woman wearing headscarves or covering her hair, for any reason, should not be allowed in public?
I've seen many a soccer mom out in public wearing a cross on her neck, which is a symbol associated with a long and painful death. Much more sinister than a head covering in my opinion. Why is this OK and not a scarf?
It's really no different than seeing a man who just got a fresh cut covering his hair with a durag. It might not be for religious reasons, but his hair is still covered for his own purposes, of his own volition.
Just the same as women wearing a hijab.