AI surface ships do have an active intercept sensor. It's just the player-controlled Perry that gets shafted on that.
Shameless plug warning: The next edition of LWAMI will feature some changes that make passive torpedoes worthwhile. Torpedoes will be quieter (especially modern electric torpedoes) and will have a steeper sound-speed relationship, and passive seeker ranges have been extended. Passive mode will probably be the preferred mode for ASUW with these changes, while active will remain optimal for ASW. Passive torps are acquiring submarines if they are loud enough, but generally won't acquire a playable sub at tactical speeds or less unless the torp runs it over.
As for CMs, active decoys have a high active source level, which means active torpedoes will tend to detect them before other targets. Passive decoys are the same, but with a high passive source level to attract passive torpedoes.
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