Two issues: 1) a judge typically does not have a degree in prison management such that he or she could make a rational decision about what type of prison is needed to protect the inmate, other prisoners, guards, and society, and 2) incarceration, the loss of liberty IS the punishment that the judge gives, full stop. We don’t put people in “real prisons” to make them “suffer.” The fact that you are forced to stay at a location, away from friends and family, and that you cannot just go down to the local pizza joint is the full punishment the state can get from you from imprisonment. Anything additional than that is solely for running an efficient prison system.
Back to the OP and BTK, yes, it is almost certainly for his own protection as he would likely last about 11 minutes in general population. Look at what happened to Whitey Bulger and he was a frail old man.