I've been in IA for 6 months, 4 of those being the Assistant Commanding Officer, and your claim is, quite frankly, gibberish. You mention that they won't be punished if they’re friends with the investigator, this is not possible - IA Command is the one that overlooks reports and sends out final responses, not the investigator. IA Command can disagree with the suggestion of the investigator. Even if they're friends with command, nothing else will happen, there are only two members of cmd in IA who are investigators.
DT 1+ and SGT 1+ have been punished before from IA reports, and I do not understand how you would have information on past IA reports on these members.