Just imagine the situation. In point X, the ASW helicopter was downed by floating SAM capsule. What could be deduced from that mere fact? Three possibilities:
A)The submarine is nearby and deployed capsule for self-defense
B) The submarine was nearby sometime in the past, and deployed capsule for some other reason (maybe A, maybe C)
C) There were no submarine here for the first place, and capsule was specifically deployed as a decoy and a way to hamper the anti-sub efforts (anti-air minefield)
Let's consider those possibilities. Unless ASW forces have additional data about the submarine, they have three scenarios with similar probability. Of those three, only A - that submarine is nearby and deployed capsule for self-defense - promise any possibility to actually catch the submarine. Both B and C scenarios gave nothing besides downed copter.
So essentially, the existence of SAM capsules force the opponent into the situation, when he is forced to risk his ASW aircraft for mere 33% probability of actually catching the submarine somewhere near the place when SAM capsule is. Considering that ASW efforts by themselves are far from foolproof - even if the submarine is actually nearby, it could still get away - the floating SAM capsules looks like a pretty good idea.