What drives people to steal precious books?

This article from the Financial Times focuses on the theft of rare books. But what about the statement that ' “inside jobs” account for upwards of 70 per cent of all library theft in Europe and 80 per cent in the US.'
How do we feel about that statement and the idea of increased internal security? '...the Library of Congress in Washington, DC, where all staff including top directors are searched whenever they leave the building'.