Airbrushing the nipples out of photos of clothing models in catalogs and on magazine covers is a strange practice indeed.
One might even go so far as to deliberately miss the point and say that it's extremely silly to protect children from the sight of a nipple seeing as it was probably the first thing they learned to recognize as babies.
More seriously this brings up a place where practicality does not mesh with our society's morals. Nipples are necessary and they exist no matter if we like it or not (and most people like them quite a bit). They help feed our young, and if that's not practical, I don't know what is. On the other hand they bring us pleasure. That's against our society's relatively Puritan morals (at least in the USA. I imagine other cultures are more sensible about them, seeing as it would be hard to be less sensible.). So we must hide them, and not let our kids know they exist, or what they're good for, because if they were to find out (I'm sure they already know) it would corrupt and destroy them for life.
Actually, this is just a ploy to see more nipples =:-)