The Placebo Effect
As you can see from the picture that Amazon offers, these sticks need to be "sticks," not little pieces, in order to not simply wash through a sink trap.  Unfortunately, the sticks are very fragile, and many of them are broken into separate inch-long pieces during shipping.  If they're not broken then, they will quickly break when handled.  I can't imagine that the pieces stay in a household drain beyond the first time water is run, and I think most people who think they work are simply imagining it.
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