I always thought this was just to do with the fact that black hair takes "doing" in the first place. I don't really hear white girls asking each other why they didn't "do" their hair, but it's so commonplace in the black community.
White women can usually wash, blow and go. If we washed daily and maintenance was simple and down to a 30-minute science, we'd probably be all wearing our hair down most days, too. White women's hair tends to grow longer with less loving care than many black women's hair, as well.
So we go to the salon to get that "carefree" look, to get curls or straight hair. And the salon was where you got your hair "done"--so to me that's where the term comes from. We don't usually come out of the salon pulling our fresh new style back, we wear it and look cute. But after a few days, it's not "done" anymore, so back it goes in the bun or pony until wash day, salon day, whatever.
That's all I ever thought it was about. Black women tend to spend a lot of time on their hair to achieve looks--we relax so that it's straight and sleek in our buns or our wraps. We curl the ends or flatiron straight. Most of us don't have the luxury of just tumbling out the shower with a wet head, running a comb through it, throwing it up with a clip and then shaking it out once we get to the office and looking great. So we "do" our hair--but I know I've got to do my hair just to get my bun or pony to look nice--but if I'm not wearing curls out, it's not "done up" all nice and fab I guess. It's just been washed and the ends smoothed and curled.