Additive versus subtractive mediums are the key here. Your computer display adds light to display things, so light text on a black background makes sense and is (for me, anyway) easier on the eyes. Printed medium is subtractive, it takes away light to display information, so a dark text on a light background makes sense for that. Perhaps you guys just need to update your prescriptions.

As per the power usage, yes, the background has no effect on backlight power usage, but some LCD types and all CRTs use less power in displaying dark pixels, so there is a positive correlation to the amount of energy saved and the use of dark backgrounds, though miniscule.