In American culture, why is a woman going topless fairly acceptable as long as her nipples aren't showing?

[Kevin]

Nudity is not at all acceptable within our general society. Americans are fully clothed when we leave the house. Why? The vast majority of us do not lead the lives of entertainers.

Edit:
You're speaking about the world of advertising and entertainment.
They do such things in hopes of shocking us in an attempt to grab our attention and hopefully sell us a product.
You assume that the practice is acceptable to most of us? I disagree.
The practice may be LEGAL but certainly it is not culturally acceptable but as the saying goes--- sex sells.

You may wish to redefine your question.
It seems that you're trying to ask "Why is it legal to show <X> form of nudity but not legal to show <Y> form of nudity? Leave off the "acceptable" bit.
In our general society, public nudity is considered offensive. Why? Victorian and religious principles we inherited from England and the Bible.