If I'm walking down the hall at work and somebody offers me a free donut, there's a pretty good chance I'd take it even if it's not my favorite kind. But put that very same donut for sale at a typical donut price, then I'd be much less likely to purchase it because likely it's not the flavor I want and/or I don't really feel like a donut at that time, at least not enough to part with cash for it.
But does eating that donut fill you up and make you decide to not buy a donut yourself when you otherwise would have? Then it's still a lost sale.
Beware of bugs in the above code; I have only proved it correct, not tried it. -- Donald Knuth