Light pollution comes from excessive artificial light. Since there's no electricity in pony world there's almost no light pollution.
They light up their houses anyways, so I am guessing either oil lamps, torches etc. So there must be SOME light pollution; those sources are artifical too :|
Anyways, no need to restrict a show to physics, that is a part of why we liek to watch stuff.
Because the characters can do things you normally can't.
