I think most people who would otherwise just decide to go topless are wary of the thousands of different situations it would create.
Can you imagine the blurred line between public streets and private business? You'd have people sitting at a restaurant calling the police because some naked lady is walking down the street next to them.
Not to mention the harassment, both from those who oppose topless women going out in public and fiendish men who would see a topless woman as window dressing.
I'm not a woman, so I can't rightly say how most women feel about this, but I think it's pretty clear that even if every woman knew they could be topless, most still wouldn't, even weather-permitting.