I'm equally confused here. Who searches "Seth Godin The Knot" and if the first result isn't that exact book, goes and buys a book about knots or weddings or something? The book should be there in plain view at the top of organic best matches. Did they really test this "yield" claim?
Probably not a lot of people. But a sizeable portion will instead go 'Huh, I guess it's not on Amazon. I'll wait until it is' and end up not buying the book.
Amazon won't care because it's peanuts for them. Sellers will care and will pay amazon more in ad revenue than it will cost in potential sales.
I believe what happens for these users is they think amazon does not have it since it's not visible on their screen or near the top and click away, look for something else.