Odd how often those tasked with enforcing the law break them.

18 U.S. Code § 700 - Desecration of the flag of the United States; penalties (1) Whoever knowingly mutilates, defaces, physically defiles, burns, maintains on the floor or ground, or tramples upon any flag of the United States shall be fined under this title or imprisoned for not more than one year, or both.

Title 4, United States Code, Chapter 1 (g) The flag should never have placed upon it, nor on any part of it, nor attached to it any mark, insignia, letter, word, figure, design, picture, or drawing of any nature.
(i) The flag should never be used for advertising purposes in any manner whatsoever.

Every officer wearing a blue line flag is guilty of desecration of the United States flag (and making political statements using their official uniform) and should be imprisoned and fined.