"JPay will get money from transactions that happen via the tablets (ebooks, music, videos)."
Convicts already extort other convicts for commissary and packages. Could be a possibility of extortion...forcing an inmate to purchase items they don't really want. All the extortionist would have to do is take the tablet to watch or listen to the products the other inmate was forced to purchase, then return the tablet to the extorted con when he's done. And it would happen over and over. Having worked 25 years in that prison system, it wouldn't surprise me if that happened.
But maybe it cause some to read books and that can be only good.
I must defer to your judgement as I know nothing about the prison system, but I add that my optimism about it comes from my own life. After I bought my Kobo reader I started to read books and I was on a much worse road than a book reader before.