If it was a valid reason, then they would have stated the reason, don't you think?
Well, Judge Reinhardt tried real hard in Silveira v Lockyer. The man did go on and on. (But he was on a mission to refute Emerson.)
He stated his reason quite succinctly. Furthermore, he acknowledged the three different interpretations of the second amendment. What he didn't state was why he believed the first clause to be limiting (ie., the RKBA is only associated with a militia), rather than simply descriptive of one possible use for arms.