They obviously don't touch on it because Oppenheimer wasn't involved in the selection of the sites or the actual bombing. He may have had an opinion on it but it would be secondary to the people who were more directly instrumental in selecting the sites and actually dropping the bombs. It's no different than suggesting Americans should be offended that Jiro offers no opinion on the attack of Pearl Harbor in The Wind Rises - fictionalized or not, both are biographies about people, not events, and you don't ask these people to commentary (through they narrative) on events they were not a part of.