The policies of the elected government in the United States are decided by the legislative branch, as bills that are passed into law. By ignoring U.S. code, bureaucrats would be violating the U.S. Constitution.
"The Congress shall have Power ... To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof" - Article 1, §8, United States Constitution.
> "The Congress shall have Power ... To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof" - Article 1, §8, United States Constitution.
Don't stop there, carry on and read Article 2: "The executive Power shall be vested in a President of the United States of America...he shall take Care that the Laws be faithfully executed". Nothing in there about anyone else taking the faithful execution of the laws upon themselves.