You don't know there was no policy on RPGs. All you know is that RPG materials weren't banned.

Right, all we know is they were not banned, the regulations against them were new after being tipped off about this "gang".
I'm not sure what nonbanned policy you are suggesting Muraski plausibly had before hand. Perhaps to secretly observe and keep tabs on any roleplaying groups? Perhaps he had regulated before where RPGs could meet and Singer had flouted these regulations? But this was never mentioned in the cited testimony or the court's recitation of facts so I think that possibility is remote. Something else?
Apparently, the staff responsible for gang activity monitoring weren't aware of the game for those two years. They didn't become aware of it until the players started recruiting additional inmates into the game with flyers.
I expect not. It looks like the letter was Muraski's first run in with RPGs in prison.
Singer was able to order lots of D&D books and have them delivered openly to his cell. Despite being a convicted murderer with a life sentence and presumably having his incoming stuff searched.
He didn't recruit through flyers though.
Neither the court decision nor the story talk about flyers, the anonymous letter mentions:
The letter expressed concern
that Singer and three other inmates were forming a D&D
gang and were trying to recruit others to join by passing
around their D&D publications and touting the “rush” they
got from playing the game.
He decided to confiscate the material and ban any further RPG activity after examining not only the gaming material, but also the recruitment flyer and a document created by the players, the contents of which we know nothing about. It would not surprise me if the staff interviewed other inmates about the group's activities as well.
We do know a little about the document, it was his 96 page home brew campaign setting.
His enthusiasm for D&D is such
that he has handwritten a ninety-six page manuscript
outlining the specific details of a “campaign setting” he
developed for use in D&D gameplay.1
1 A typical D&D game is made up of an “adventure,” or single
story that players develop as a group. A related series of games
and adventures becomes a “campaign.” The fictional locations
in which the adventures and campaigns take place—ranging
in size and complexity from cities to entire universes—are called
“campaign settings.” For more information about D&D
and D&D gameplay, see Wizards of the Coast, What is
D&D?,
What Is D&D?
(last visited Jan. 20, 2010).
And again, no flyer.
Perhaps he only concluded the threat was worthy of an outright ban after he investigated the situation. It's also possible that RPGs had never been an issue in that particular facility before, but other games or activies of a similar nature had caused problems in the past and those experiences shaped the policy on RPGs.
I assume that the prison official has experience with inmates playing games in prison because I find it impossible to believe that he was involved in security at a prison for 20 years and never encountered inmates engaging in recreational games. I also assume that, even if he has never encountered inmates playing RPGs before, he is capable of extrapolating his previous experience in a way that allows him to assess the implications of RPGs in a prison setting and make informed judgements about the security risks they might pose. I also assume that his investigation into the specifics of this groups activities informed his policy decisions.
As for the rest, I don't assume any of it, so I feel no need to defend it.
His testimony was not that anything Singer's group was doing was different than any other D&D group or that they actually posed a threat. He said D&D in general promotes X which can lead to problems. His testimony was not that there was a new problem posed by Singer's gang, it was that RPGs are a threat issue in prison, period.
If so and RPGs had never been an issue before then wouldn't it be because he hadn't dealt with them before?
This is a good reason IMO to assume he didn't have direct experience with them before the incident.
If there was something from this particular group and the search and potential interviews that was relevant to support the ban which was being constitutionally challenged in court I would expect him to testify about that and the court to comment on the issue.
Since there was no such cited testimony I expect there was none.
It looks like all they found was D&D books and guys playing D&D.
This leads me to believe this was Murkaski's first dealing with RPGs in prison.
This leads me back to wondering the basis for his conclusion.
Extrapolating from other somewhat analogous things prisoners do that he has had experience with (reading fantasy, theatre/improv, video games(?), board games, chess, competitive sports, gambling, writing fantasy, movies, TV, anything with a referee, judge, or leader, etc.)?
His gut evaluation based on his background in security through the lens of years of focusing on and dealing with gangs and cults?
His gut view of RPGs?
Information on RPGs from other groups?
Actual studies?
Standards and practices at other prisons?
Something personal with Singer?