The director actually pitched James Hetfield to be in the movie and feels that he can bring his natural charisma and magnetism to the dramatic role.
So, clearly the director has a working relationship with the band and saw something in James Hetfield that makes him right for the role of Bob Hayward.
While this might seem like an out-of-left-field choice, there is actually already a connection between James Hetfield and Extremely Wicked, Shockingly Evil and Vile director, Joe Berlinger.
The Oscar-nominated director, who has a ton of documentary credits to his name, directed and produced the Metallica: Some Kind of Monster documentary along with Bruce Sinofsky.