The scary part about so much information about people being in a single place is that the government knows where to look. This is a good thing when they are investigating a suspect but when they start to investigate the reading habits of people who are not under any suspicion, it becomes troublesome.
In short, Amazon was asked by the government to hand over which products were purchased by a large group of people in the hopes that one or more of these customers would testify against the seller (in this case Amazon did not sell the products directly — they were purchased through Amazon Marketplace).
Amazon refused. The government narrowed the list to 230 — Amazon refused again.
The court records regarding this case were recently unsealed. More information can be found here.