I don't think they forgot that part. They just realized that it needs to be constructed as two different topics... topic 1, "a well regulated militia being necessary to the security of a free state", topic 2 "the right of the people to keep and bear arms shall not be infringed".
What the SCOTUS has finally affirmed is this disjunction; the people need not, and never needed to be, engaged in a militia in order to bear arms.