How many cartons of cigarettes can I take on a domestic flight to Florida?
Indian (or native American if you prefer) reservations are either not subject to us law or it applies differently to them (they are recognized as a separate entity similar to Vatican city in Italy) I don't pretend to know the details but that's the 100,000 foot view of their status.
That's why the smokes are cheaper there because they are not subject to the same taxation as the surrounding area (state/country).