I think it is good to introduce Dharma to prisoners as I feel it is the only thing that will be able to save them. Although the karma is already there for whatever actions they did for themselves to end up in prison, at least, through the Dharma, imprints can be created for them to be triggered in the prisoner's future lives to receive the blessings and benefits of learning the Dharma in prison.
Although not the best situation, being in prison is conducive enough for a prisoner to read Dharma books, and it could end up being their solace and source of comfort. The Dharma can take away their bitterness, their suffering, their pain, anger, hatred, for everything is in the mind. Dharma is all about self transformation, and from that comes one's own rehabilitation. Dharma can really really help them, and us!!