from:
www4.law.cornell.edu/uscode/html/uscode10/usc_sec_10_00000311----000-.htmlUSC Title 10, Subtitle A, Part I, Chapter 13, Section 311 (Militia Composition and Classes)(a) The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard.
(b) The classes of the militia are— (1) the organized militia, which consists of the National Guard and the Naval Militia; and
(2) the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval MilitiaBy Legal Definition: a Neighborhood Watch is a Militia. albeit an "unorganized" one. Section 1 and (a.) go to great detail defining a smaller subset of what constitutes an Organized Militia.
But then instead of leaving it there and having it completely exclusive to the National Guard, the law includes everyone else as part of the Unorganized Militia who do not fall into the first category.
This is like Set Theory: Define some Set A, then define Set B as the set of all objects NOT in A. Then define the Universal Set (or Militia Set) as {A U B}