This is not a good comparison. In the cases you mentioned, there is skin exposed. I agree that a wife-beater tank top or an extremely short skirt might be inappropriate in certain venues, but if she goes without a bra, it does not imply that her breasts are out there for all to see, just... unsupported.

I have to agree that it seems like an entirely personal choice. Unless she's wearing a cheesecloth blouse or a chainmail shirt, I'm not sure that any rules of decency have been violated; just the rules of slavish conformity. :-)