Netflix cares about your privacy and has agreed that if you unsubscribe from the site, any rental activity on your account that is more than a year old will no longer be tied to you, meaning no one - not even Netflix - will be able to look back at your past movie choices, and after you've been gone from Netflix for a year there will be no way to pull any rental data. Okay, the latter part of that is true, but Netflix doesn't care about your privacy.
Instead, the company was pressured to make this decision as part of a class-action lawsuit. Because the site was violating a law known as the Video Privacy Protection Act, it was forced to settle the major lawsuit for $9 million. The vast majority of that went to privacy organizations that spearheaded the legal action, but in addition to the money Netflix agreed to change its practives.
The law in question was passed in 1988 and every time it comes up in the news, it always gets a footnote that the only reason it was entered into legislation in the first place was because Supreme Court nominee Robert Bork's movie rental history (which included some off-color titles) were published in a local newspaper. Netflix is no stranger to the law and has lobbyists actively trying to get it scrubbed from the books. For now, though, it has to play by the rules.