Wait, wait, wait.
Why? It doesn't seem like they want to be topless, it seems like they want the option to be topless. A guy walking around with no shirt is different than a girl walking around with no shirt, the reason being that men (over the age of 13) are naturally attracted to women's breasts. Sure, if a really buff, attractive guy went walking down the street topless (eg Brad Pitt) it would probably catch the attention of a few women at least, but if an attractive woman did the same thing (walking down a street topless) she would turn everyone's heads.
This seems like more trouble than it's worth. Far-fetched though this may sound, women walking around topless could be beyond distracting- it could be physically intrusive. I'm talking about people driving cars getting distracted by a hot topless woman. It only takes a second of distraction to make an accident and cause damage to property and people. Plus, there's the whole "religiously immoral" thing: America is still overwhelmingly populated by Christians, the majority of whom don't really approve of this thing. Then you have to consider the aspect that many parents don't want their children exposed to this kind of thing because it could turn their sweet 9-year old sunshine boy into a horny prepubescent monster that expresses itself by raping its younger sister. Probably a bit hyperbolic, but I think my point is carried across by it.
NOTE: I don't have any personal problem with this, but the majority of Americans just don't want to see this going on and it would be controversial, if not outright harmful, if Congress actually passed this.
NOTE2: I think I have some more posts about this on the old thread about nudity. Can't be arsed to go looking for it, though.