Yep, it couldn't be more transparent. The push to take away mail and books from inmates and provide electronic versions for a fee is worth a ton to vendors because, well, they have a literally captive audience. (Similarly, extortion type rates are in place at prisons that charge to have phone calls with loved ones, and is why there's an effort to end charges for these calls. )

Prison labor pays almost nothing (and it is basically slave labor) so this is to squeeze the families, mostly. And exert control.