Excuse me for spoiling the fun with a semi-serious answer.
(And I haven't really spent time in a mall in years, so some of my observations
may be out of date.)
You might notice that mannequins these days also have facial features
(noses, eyes, hairstyles, muscle tone) that older types of "store dolls"
never had.
In the "old days" mannequins were just used to hang clothes on so customers
could see what the clothes looked like from outside the store. But, at some
point the fashion designers decided that people might want a better idea
of how the clothes would look on a more human body - so the mannequins started
getting more humanlike features. And since retailers cannot afford to hire
supermodels for every store, it became easier to model the dolls on the
body types of "the beautiful people". Also, there is a trend to pose the
dolls in ways that imply a more active lifestyle, as if at the end of the
day the mannequins are all going to march down to Starbucks for a
double mocha latte. Hence why most store mannequins these days look like
life-size Ken and Barbie dolls.
Why the nipples though?
Marketing Secret 101:
Sex sells, but blatant sexuality is a turn off (for most people).
Because of the above "rule", whenever you are trying to market a product,
you want to give the impression that what the customer sees is "sexy", without
actually making the customer think about sex. One of the ways to do that
is by using some of the secondary sexual characteristics of the human body.
These are features on humans that might be sexual signals, but can usually
also be recognized as signs of excited stimulation, (or just cold).
The secondary characteristics provide just a hint of sex, enough
to get noticed without being blatant.
Nipples are one (two?) of the easiest markers to use in this role, because
(in our progressive society) most adults know enough to not automatically
associate them with sex, but still manage to associate them with excitement,
and so associate the clothes with excitement as well.
The next time you are in the mall, try to notice these other features:
Do all the mannequins seem to be thin and "perky", implying youth
and energy?
Are the mouths on the faces closed, or slightly parted - again implying
excitement and an active lifestyle.
Are the eyes on the mannequins closed or open? And what kind of expression
do they seem to have?
Do the clothes really look that good, or is it just the way they are put
on the dolls?
Can you tell I watch waaay to much of The Learning Channel?
Toolgirl, I think that some stores use naked mannequins to shock shoppers into
noticing something different about that particular store.