There was a whole aisle of them at the Leslie's I went to. If they did what they claimed for $35, Leslie's wouldn't be selling them for robbing the incredibly profitable business from the rest of the aisles.
However. They do the opposite and help sell the other products. The staining if it goes that long helps sell all the metal outs and sequesterants. The inadequate chlorine levels used, even worse than their existing inadequate levels, fuel every other product they sell like the shocks, floccs, algecide and yellow out.
They know exactly what they're doing.