Out of curiousity, and respectfully - who are you? You've been on this site for four years, and yet you've only made a handful of posts. I'm sure there's a story behind that.
To answer the question, it's more than just that, I think. Allowing D&D is the potential for weapons - hardcover books, dice, and whatnot can all become dangerous weapons. So, if your prison doesn't allow boardgames, it might not allow this. As has already been mentioned.
As for violence, disconnect from reality - I remember the gov't of sweden doing a study on RPGs that concluded the exact opposite. It's online, somewhere. Also, Michael Stackpole's "Pulling Report", while a classic, is always a great source to use. Finally, doesn't Yale have a D&D class that offers credits? I remember hearing something about that. I think you can always use the "if it works for Yale..." argument, there.
I think a great way to describe the game would simply to describe it as a social gathering of players, using pre-recorded information. Almost akin to a book club, really, just a bit more interactive. And with dice (and there are methods to go diceless, by the way).
All that being said, if he's not allowed D&D, that's fine. I personally don't see a huge problem with that - prisons aren't country clubs, and if the powers that be decide it's not safe playing D&D, knowing the facts, that's for them to decide. After all, there are plenty of harmful effects D&D can have in a prison:
* It causes an authority chain (responsibility to the DM, for example - I can see this leading to a 'gang mentality'.) If someone threatens my DM, I have to protect him. And my group of gamer buddies take up a large portion of my time, and we probably become a clique. And it's a group that has a leader (a DM). Now, it might be farfetched, but if I'm in a prison, I probably have some problems with decision-making - if my DM says "hey, guys, King Pooba in Cell Block D is ticking me off.... shank him in the showers and you all get 1,000 XP"...
Kind of a very nerdy gang, but still something worrisome for prison guards.
* The aforementioned weapon aspect. A single PHB can be turned into a knife, due to the very hard cover. Role-playing without books and dice is possible, but it might be a bit difficult unless your client is rather creative.
* Protection of the players. A lot of prisoners are, lets face it, bullies. And if you see a bunch of people playing a game that is rather universally seen as a geeks game... it could lead to trouble for the people playing it. After all, the other prisoners are bored, angry, and could easily see a potential target in someone playing the game.