Most of the scenes that take place in public are shown during the day, so it might just be that the male ponies are all working while the episodes take place. Most of the ponies that we see in the background look a bit younger than the main cast, at that, and regardless of what
Word Of God says, I still feel that they're at least in early adulthood, so one could easily assume that the background characters are just doing as teenaged girls are wont to do, roaming the streets.
Backpeddling somewhat to the conversation about Luna (Not my fault you people update so fast), I've been thinking about that and...I really, sincerely hope that

Nightmare moon and princess Luna aren't one and the same. Partly because having the rainbow just help Luna shake off a nasty little magical symbiote almost guarantees epic plots in the future, but mostly because if that isn't the case, that would mean that it did something towards stripping some of her goddess-magic from her, or worse yet,
buggered about in her brain box. It might just be me, but knowing that Celestia (and Twilight, but she concerns me less) presumably has access to what could either turn out to be a god-killer or a tool for mass-brainwashing...brr...