A long, long time ago SCOTUS ruled in a number of cases that prisons could restrict erotica (as well as outright pornography) if its presence in the facility could be shown to have a negative impact on security and the orderely operation of the facility. (Pretty close paraphrasing of the essential comments.)
Basically, if porn could be passed around and used to reward/control behavior (join a gang and read porn, shank someone and get pictures, etc.) then the prison had a legitimate reason for controlling/prohibitting it. Pelican Bay is what the system I worked in would call a Level 5 facility - supermax segregation with little if any contact between one inmate and another. The possibility of passing any contraband between one inmate and another (without the complicity of staff) is small to none.
When you get locked up you lose certain liberties - like the liberty to come and go at your own pleasure, your liberty to live where you want to, your liberty to choose your own clothes, and the like. Unless your choice of literature involves plans/descriptions on how to make weapons or escape tools, or would incite other prisoners if passed among them, or similar dangers an inmate does not lose his right to have literature to help enhance his fantasy life.
The question never should have come up, and once it did it should not have taken 2 years to get resolved.
stay safe.
