In Miller in re militia
It’s about the militia of the states, barring federal action but not state or private action.
Court observed that ”[w]ith obvious purpose to assure the continuation and render possible the effectiveness of such forces the declaration and guarantee of the Second Amendment were made. It must be interpreted with that end in view.”5 The significance of the militia, the Court continued, was that it was composed of ”civilians primarily, soldiers on occasion.” It was upon this force that the States could rely for defense and securing of the laws, on a force that ”comprised all males physically capable of acting in concert for the common defense,” who, ”when called for service . . . were expected to appear bearing arms supplied by themselves and of the kind in common use at the time.”
So, in order to make it possible for men to report for duty with weapons. In other words, for them to be able to fulfill their responsibility as militia members, it’s a right of membership in the society.