Nate Jones of The Washington Post discovered a Freedom of Information Act request from Muckraker that sought to find out if federal prisoners are allowed to purchase Dungeons & Dragons and other roleplaying games. The answer is no, but the any reasons for the ban are redacted.
From Reason:
Any rationale for the ban, if given, is buried under redactions, but D&D and other roleplaying games are widely banned in state prison systems under the dubious rationale that they present a security threat or encourage gang behavior.
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It's all silly, but it does illustrate how counterproductive and dumb prison book bans can be. Of all the things you could be doing in prison, D&D is one of the better and less offensive ways to pass the time. It's social, encourages teamwork and empathy, and as one former incarcerated man told me, gives "the vilified an opportunity to be the 'Good Guy' that the world in which we live rarely does."