You are not what you read: librarians purge user data to protect privacy
“I was approached years ago at a different library about users who’d checked out certain astrological books,” said Thistlethwaite. The NYPD officer told her he was looking for the Zodiac killer. “Most police investigations are a little smarter than that, but sometimes they’re just not.”
Seems pretty clear to me: one of the principles in the Data Protection Act is that data should not be kept longer than is necessary. Admittedly this is a news article from the US, where there’s no direct equivalent of the DPA, but still.