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

[bleuflamenco]

Like Nicki Minaj's Instagram selfies, for example.
Kevin, yes it is not really considered acceptable in every day situations, but it is displayed on magazine covers, billboards, and the such and is not considered as offensive as a woman having her breasts fairly well covered, yet a nipple is peeking out from a top.

[Miss Awesome]

I don't know. To be honest, I think it's silly that breasts are considered perverse at all.

[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.

[Mark]

Because people like to push boundaries. It's also why full dorsal nudity is common on TV, but full frontal nudity isn't.

What's even funnier is this behaviour has gone on for hundreds of years, not just in the US. Women in England in the 1700s used to wear kind of thin dresses, wet, so you could more or less see everything, but they could fall back on "I'm covered!" if someone objected to it.

[calivette]

Thats one of the dumb parts of American culture. A mans nipple and a womans nipple is the same thing but we have to pretend that a womans is something totally different.

[Jennifer]

In direct answer to your question those were probably glamour shots for magazines. In most of the glamor shots you see you really don't know how much of a girls breast you're looking at until you see the nipple. She could have it all out there down the last legal inch as I call it and all you see is skin. Actually the beach ordinances in most communities require a woman to cover her breast below the top of the areola, which the dark area around the nipple itself, in other words the lower half of the breast. Now as a rule as long as a girl makes an attempt to keep the nipple and a portion of the breast covered the life guards/police look the other way. Even if 3/4 or her breast is exposed sunbathing people have a tendency to look the other way as long as the the bikini top is partially there.