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

[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.