Once they do the background check to grant a clearance, there has to be a certain level of trust given to the clearance holder. Of course, there are those who violate the trust for reasons both good and ill. Risk assessment, I suppose.
Way back when, we were forbidden from bringing Bamford's book "The Puzzle Palace" into secure areas. Although NSA refused to confirm or deny anything in the book, the fear was that people may bring it into the facility and make annotations, and it all goes downhill from there. Young servicemembers busted for possessing classified outside secure areas, blah blah blah.
It may seem silly from one perspective, but it's logical from another.