Never use metaphors for online issues. They simply don't apply, the online world has far too many differences from the physical world for them to ever work. It isn't like "leaving books around a library", it is like "digital online content was accidentally priced for free when it shouldn't have been and some people downloaded them". This isn't such a complicated situation that we need metaphors to navigate the morass, not that they ever help anyhow since it inevitably causes people to get distracted by arguing over which inapplicable metaphor is more accurate instead of simply addressing the question at hand.
What really matter here is the breach of contract. The contract specified when Amazon can drop the price, that condition was not met, they dropped the price. Apologies or blaming computers are not what is called for here.