"Why do you need an AR15?"

[stanc]

A 17 year old male is in the Unorganized Militia (10 USC 246) and, if called to duty, must appear with a weapon of his own procurement (U.S. v. Miller, 307 U.S. 174 (1939))...

The sentence in Miller to which you are referring is only part of the notes on the history of militias; it is not a statement of what is required by law.
The requirement to appear for duty "with a weapon of his own procurement" was eliminated when the Militia Act of 1903 superseded earlier versions.