Earlier quoted context omitted.
Yes, this seems needlessly restrictive. What's the harm in letting someone with nothing better to do read Harry Potter? I could imagine restrictions on certain kinds of books... like how to cover up forensic evidence, or how to tie knots, or whatever... but Harry Potter, or computer programming?
There is s legitimate safety concern. That is certain parts of books can be turned into weapons. But the real issue is one of control. Treating prisoners as subhuman is a management practice.
Slight correction: Treating prisoners as subhuman is a societal practice. (At least, in the societal context that this discussion and the article are about.)