To add to this thread and the one Augie closed:
A white wire only becomes a neutral when it has been connected as such; otherwise, it's merely a wire that happens to have white insulation. The extensive presumption that a white wire is always a grounded conductor is why the code evolved to require it be re-colored whenever used as a non-grounded conductor.
When BX was popular, it was common to supply the ceiling box, and feed receptacles and the switch loop from that box, so you'd have several whites with a light pigtail, several blacks with one white, and the return black for just the light. The blacks-and-one-white joint would normally never be disturbed in the future.
Any experienced electrician would recognize every connection he might encounter (as long as the wiring was done correctly), but a DIYer or handyman may expect every white wire to be grounded, so the code has accommodated them. Plus, electronic controls weren't around then; code evolves as equipment does.
