They mean that the existence of prisons, the criminalization and mass incarceration of populations, etc. are all inherently political processes. They may intend by consequence that the solution to the problem of prison is also political. It is akin to the feminist slogan "the personal is political"--the claim being that gender dynamics between romantic partners, the distribution of household chores, instances of domestic violence and sexual assault, and other supposedly personal issues are political.
Fred Hampton Jr. and Prisoners of Conscience Committee: "All prisoners are political."
Austerity Is Prison / Bay of Rage: "This is why we say that all prisoners are political prisoners, their incarceration the product of the machinations of power, the flows of capital, and the structural prejudices of the police."
Myriam Denov: "For some people, all crime and subsequently all prisoners are political, because the reaction of the state to crime is largely in the interests, values, and beliefs of the law-making power and ruling class."
Mostly it is wishful thinking and empty rhetoric. But it also sheds light on a political program for the abolition of prisons which involves, in part, the politicization of prisoners.
Interesting questions that are raised here are what 'political' means, and what relationship the political bears to the existence, abolition, or destruction of the prison society.
I'm reminded of the following part from "Take Your Mark, Get Ready, Ablate: 3 Positions Against Prison" from Fire to the Prisons issue 10. The author is making an argument against what they refer to as a leftist political movement aimed at the gradual abolition of prisons, and in favor of the immediate destruction of prison society through revolt both inside and outside of the walls.
1. There are no political prisoners, only prisoners of war.
There exists a third definition of political prisoners. As the movement for prison abolition has grown on the Left, there has been a tendency to radically expand the bounds of who are designated as political prisoners. And a radical new phrasing has been inscribed in the pages of the Leftist Bible: "All prisoners are political." It is a kind gesture, but only because it is made by people for whom the label 'political' is a compliment. Perhaps we should have first asked the prisoners if they wanted to be political. What, and stop saying 'bitch'? What word could be more degrading than 'political' to apply to people without their consent?
This tendency seems to overlook that the original reason for describing some prisoners as political was to illuminate our bonds of affinity--to identify prisoners of a war that we are fighting on the same side of. There are Nazis behind those walls. Let them free, certainly--the better to crack their skulls--but surely we can express our desires without expressing solidarity with our enemies.
[edited for italics and block quote]