Weston's headless nudes were made that way because it was an integral part of his composition. This emphasized the torso (much like the Venus De Milo) by eliminating the face which is the natural focal point of a picture.
A headless nude is a ridiculous affectation. It ignores the fact that the De Milo actually had a head. Why should an accident of history become an art standard.

Something else that is ignored is that greek/roman statues were white. They were actually painted to make them appear lifelike.