The essential issue is that, for the AI, the first turn and decoy drop are the crucial maneover for them to evade the torpedo. If they do that maneover correctly and drop the active decoy they can evade even very close shots, however, if they do the correct maneover and drop a passive decoy, their chances of survival decrease dramatically, even given the fact that sometimes the passive decoys will occasionally spoof an active torpedo.
In the same situation, for a passive torpedo, due to the combination of shorter seeker range, slow snaking pattern, and slow closure speed, the first evasion move alone is almost always good enough to evade the torpedo, even without a passive CM.
So, in my opinion, it is about making the AI take the correct action against the most dangerous threat to its survival, and that is an active torpedo attack at close range, and if it drops a passive decoy, as it too often does, then the game is up for that submarine in most cases.
Like I said, I've stopped carrying passive decoys on my dives, because I don't fear being sunk by a passive torpedo, so I think the AI captains would make the same decision.
