What the song engaged with, in 1955, was a cultural imagination of prison that was very much present in popular culture. Prison films were a significant genre; prison songs were part of the country and blues tradition; the experience of incarceration, however statistically uncommon for most Americans, was culturally visible in ways that made a song about a Folsom prisoner legible to a broad audience.
By 1968, when Cash performed the song at Folsom and recorded it for release, the cultural context had shifted in ways that gave the song different weight. The civil rights movement had brought questions of criminal justice and incarceration to wider attention. The disproportionate incarceration of Black Americans — documented in NAACP and other civil rights organization reports from that period — was part of the political conversation in ways it had not been in 1955. Cash’s advocacy for prisoners, his stated belief that people in prison deserved better conditions and more opportunity for rehabilitation, was not disconnected from that broader context even if he did not always articulate the specific racial dimensions of the issue.
The song’s narrator, it should be noted, is not given a race. Cash sang it in his own voice, with his own face. The audiences who heard it — including the Folsom audience, which was predominantly, though not exclusively, comprised of men of color — heard a white country singer identifying imaginatively with a prisoner in a maximum-security facility. Whether that identification was understood as genuine or as appropriation, or as both simultaneously, varied by listener.