That's because libraries, like bookstores, are equipped with anti-theft systems. Every book has a thin magnetic stripe that will be sandwiched in any page. If it is not scanned by a bar code reader, it will not be demagnetized. If it is taken away from the exit, it will call the police. Different from bookstores, books in bookstores use disposable magnetic strips, which are demagnetized after purchase, while magnetic strips in libraries are permanent and reusable.