After reading the article, it's about what I expected. Games force you to make quick decisions, usually, and it makes sense that practicing that ability would lead to an improvement in that skill in real life.
I do think there are ages at which the violence can absolutely affect the mental processes. Past a certain point, however, I think once those neural pathways are (mostly) set, you're probably just fine. I've played my share of violent games, but I also think I'm pretty well grounded by this point.
I think, with the prisoners, a lot of those guys have those mental processes already established, and it's hard to pull them out of it. The problem with living a certain lifestyle, no matter how good or bad it is, is that it's self-enforcing neurally. It's why "habit" is so hard to break, as the brain's "hardware" has literally changed to accommodate that behavior. I can't see giving them violent games to continue the enforcement of those mental processes, but I also don't really think "good" games will help much, either. It takes something a good deal more powerful than that to really turn those hardened criminals around. You might have some luck with minor offenders, though.