Those who advance that augment, do so by attaching a modern big-government meaning to the word regulated. The word has another meaning which was more prevalent during colonial times. That other definition is:
[adj] marked by system or regularity or discipline; "a quiet ordered house"; "an orderly universe"; "a well regulated life"Those were times when all order, discipline and regularity was not established by outside agencies. It was a time when self discipline, regulation and order were the norm. According to this meaning, they actually meant an orderly and disciplined Militia, which required members who already owned and knew had to to use firearms.
Additionally, I don't see why some assume that the 18th Century meaning of the word "Militia" meant only the State Militias we see today. I've read of much smaller and local groups of armed citizens, from a town, region, territory etc., being refered to as "Militias" throughout American history. The "security of a free State" was not left only to State Militias.
The problem is that Militia means exactly what atleast 5 members of SCOTUS want it to mean.