Enforce and protect United States Code, specifically 10 USC, Subtitle A, Part II, Chapter 45, Sections 771 and 772 which bans wearing of the United States Military uniforms by people not on active duty are published in the United States Code (USC).
Specifically, 10 USC, Subtitle A, Part II, Chapter 45, Sections 771 and 772.
Section 771 states:
Except as otherwise provided by law, no person except a member of the Army, Navy, Air Force, or Marine Corps, as the case may be, may wear -
(1) the uniform, or a distinctive part of the uniform, of the Army, Navy, Air Force, or Marine Corps; or
(2) a uniform any part of which is similar to a distinctive part of the uniform of the Army, Navy, Air Force, or Marine Corps.