I could understand him being guarded about it in the 80's and maybe even mid 90's, sure. But now? His place in history has been cemented for the last what, 20-30 years? He doesn't have to worry about anything. Nobody is usurping James Hetfield for his contributions to metal. Even if he put people in a time machine and plugged them into his old rigs, few people if anybody would sound like him. That right hand is doing a lot of the work. Most every metal guitarist knows how to play Master of Puppets but have to ever tried to record yourself playing that stuff, listening back to back to the real thing, and really try to nail it? It's far from easy.
I guess I'm just saying even if he did give away every secret he had, most people still wouldn't be able to play just like he did. At this point I think it would just the courteous thing for him to do, hah.