By paying nothing for it, you are telling CAH that that is exactly how much you think their product is worth to you. Nothing. If I were them, I would be a little bit hurt to see that.
Key point: "to you". I imagine that if the $0 option wasn't included, many of those who chose it would simply not "buy" it at all. So at least they get some marketing benefit out of it. Of course this works better for those "pay what you want" digital goods, where there isn't much marginal cost to giving away a copy.
