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?
Also keep in mind that guards are often the ones smuggling in comfort items and drugs. I imagine they don't like it when someone is sending prisoners things for free that they would otherwise have to get through the black markets in the prisons.
>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.)