A long time ago I delivered pizzas. You'd get people who would call up and claim something was wrong with the pizza. We just sent out a new one and didn't ask for the old one back, or for any sort of proof of their claim.
You're probably not gonna be in financial hardship if you never sold another copy of the book, but the principle still applies: The customer is always right.
The boss basically came to the conclusion that the amount of money he would lose by treating a customer with suspicion FAR outweighs the amount of money he'd lose sending out one free pie. Most complaints, fair or not, were from people who genuinely felt wronged, not scammers trying to score something for nothing.