It's rather amazing, to me at least, how many times we have to explain this. It's not a slam on any individual, more it's a slam on our educational system, that the founding documents of this nation are so well hidden from (students) view. sigh.
Back in the 18th century and for much of the 19th century, the English Language was bereft of standardized punctuation (as we know it today). It was more of an individualized stylistic device. Therefore, to try to use punctuation as a means to understanding the words, is at best a faulty premise to begin with. At worst, it's plain futile.
[i]"A well regulated militia, being necessary to the security of a free state,"[i] is a prefatory clause and of and by itself, an incomplete statement. A sentence fragment, as it cannot stand alone, without adding some wording to complete the thought.
"The right of the people to keep and bears arms, shall not be infringed," is however, a complete thought and stands alone as a complete sentence.
The nature of crafting a prefatory clause to such a statement, merely provides one reasoning (justification) behind the function of the statement. Is it the most important reason for the operative clause? That's a debatable point.
Law Professor Eugene Volokh, has written an excellent paper on the subject of prefatory clauses, their uses in Colonial law and legal writings. You can read it
here.
The operative clause, on the other hand, emcompasses the actual right, and was written to prohibit the national government. The legal debate will, in time, detail the scope of this right, as it is applied to its citizens and what prohibitions it contains to the government.
What is not debatable, at this point in time, is that the Supreme Court has decoupled the prefatory clause from the operative clause.