For those unconcerned about the NSA activities…
…here is a lesson in Data Mining 101:
This is what’s possible with publicly available information, but imagine if one had access to Amazon’s entire database – which still contains every sale dating back to 1999 by the way. Under Section 251 of the Patriot Act, the FBI can require Amazon to turn over its records, without probable cause, for an “authorized investigation . . . to protect against international terrorism or clandestine intelligence activities.” Amazon is forbidden to disclose that they have turned over any records, so that you would never know that the government is keeping records of your book purchases. And obviously it is quite simple to crossreference this info with data available in other databases.
I suggest you read it all and do the math.