Specifically they were concerned because some inmates have access to computer systems and they feel that this book is a security threat because obviously if you know anything about Java you can hack any computer anywhere.

Well, learning a programming language on its own probably won't help you probe digital security very well, but these are very motivated individuals, who have access, as you say, to the prison computer systems; who, if they're sharp enough, might find some exploit--especially if they have all the time in the world to work at it. I still think its not great, I mean prisons in general aren't great. But if I'm being honest, my motivation for giving prisoners access to reference books about software and computer architecture would be at least partially to help them break out, I think my bias here says everything.

A prisoner hacking prison computer systems is simply providing free pentesting services.