I actually find this data interesting, as long as it's anonymous. It should probably be something you have to opt-in for though.
Even presuming some dystopian future where The Government (cue hammy antagonist music) is busting your chops for what you read, the fact that Amazon has a receipt for you purchasing a copy is likely to be a bigger deal to them than which bit you highlighted.
Not all of Amazon's customers live in the U.S., either. It appears that they don't currently sell the Kindle in Iran and China, for example, but if they did, I sure wouldn't want to own one there.