I think the 6 week thing is extremely conservative and wise. Women around the world (traditional societies, with close female relaties nearby) follow a 40 day rest rule. Mothers are not allowed to do any strenuous work... female relatives cook, clean, make nutritious meals for the new mother.
Some new mothers (depending on their country/customs) get special massages to help healing along.
Postpartum care in Ancient IndiaMaternity Wise - Traditions - from Robin Lim's book
I can't find it online anymore, but I read a study once of Indian rural women who went back to work during the first 6 weeks, and they found more instances of prolapse.

Another link I can't find... In Vietnam women are not allowed out of bed because they fear the uterus falling out (rest, rest, rest, heal, heal, heal.)
Sex is wonderful, but not particularly restful, especially in the first 6 weeks (personally speaking.)
So... IMHO simply abstaining from sex (anything strenuous) is a good idea for most women.

though I know people who felt "just fine" within days and were up and about doing stuff.

Not me... I felt my insides coming out and I needed bedrest.
So from personal experience, I would advise women to * really * take it easy the first 6 weeks.
A baby just came through that birth canal and needs time to heal...
American childbirth educators or midwives... aren't aware of all the world practices, are they? I really believe this attitude (self-sufficiency & independence - women refusing to lay in bed and rest because they feel fine - that was me the first time) is cultural. I'd love to hear the opinion of a Mexican or other
non-Western midwife on this subject.

Wow I just found this totally OT
University of Michigan - Health & Cultural Beliefs