Journalist Shane Bauer, who spent two years in detention in Iran, has produced a series of articles for Mother Jones about prison and solitary confinement. He's fiercely critical about it. Here's his video introduction to the articles.
His articles argue persuasively that the assignment of many inmates to solitary confinement is irrational, unrelated to their actions or conduct, and frequently based upon questionable evidence. Unfortunately, he makes the assumption that what prison inmates tell him is trustworthy: and therein lies the Achilles heel of his articles.
You see, I was a prison chaplain, both part-time and full-time, for about a decade in total. In Federal and State high-, medium- and low-security institutions, I've visited inmates in SHU, in solitary confinement, etc. I've spoken with them, lived with them, had every opportunity to observe them. I'm here to tell you, the vast majority of them are not to be trusted. That doesn't mean that there are no trustworthy individuals among them; it just means there aren't many of them. Personally, I'd say at least 95 out of 100 high-security inmates are completely untrustworthy. (The proportion may be better among lower-security-classification inmates.)
I've seen people who are in solitary confinement because of threats they made to other inmates and/or prison staff; due to mental illness; owing to conduct that threatens prison security. They'll almost all deny indignantly that they did anything to deserve their solitary confinement, but those of us who work with such people know better. I've personally sent inmates to SHU for various infractions committed in my presence, or in my area of responsibility. I've responded to incidents where inmates assaulted and attempted to murder other inmates and prison staff. I can assure you, on the basis of my experience, that the majority - probably the vast majority - of those in SHU, and in solitary confinement, are there thanks to their own choices and actions. (For a particularly notorious example, see the case of Thomas Silverstein - and before you get too sympathetic for him in his 'plight', you might want to say a prayer for the souls of inmates Danny Atwell, Robert Chappelle and Raymond Smith, and correctional officer Merle Clutts. All four were murdered by Silverstein during his incarceration. Innocent as a choirboy, he ain't!)
I won't dispute Mr. Bauer's claim that in the California prison system, there may be 'broken' administrative processes that cause inmates to be put into solitary confinement for the wrong (or no) reasons, or prevent their cases from being re-examined. I've never worked in that state's prison system, so I have no first-hand experience from which to support or oppose his position. On the other hand, in the Federal and state prisons to which I have been exposed, I've seen very little evidence of capricious, wilful incarceration of inmates in solitary confinement. In every case I saw, or of which I was aware, there were either good reasons for it, confirmed by subsequent investigation, or it was reversed in fairly short order when those investigations didn't turn up corroborating evidence.
As for whether solitary confinement is 'cruel and unusual punishment', what alternative is there? Those placed in solitary have already been convicted of criminal offenses - some of them multiple times - and sentenced to long terms behind bars. Many will have 'regressed' from easier, more lenient facilities to higher-security units on the basis of their behavior. (Only the really bad boys are assigned to maximum- or high-security prisons, because it costs a lot more per inmate to keep them there.) If, even there, they break prison regulations and pose a threat to good order and discipline, what measures are left to the staff? Find me a workable alternative to solitary confinement, one that will do at least as much to contribute to good order and discipline in the prison, and I'll support it - but I can't think of one, and I don't think you can either (apart from execution, of course, and that's not going to happen).
I recommend reading Mr. Bauer's articles. They offer a sobering and painful perspective on the reality of solitary confinement in at least one US prison system. I'll even agree with him that the practice may be cruel, even vindictive at times. However, that doesn't mean that it's not necessary. It's a bit like surgery. No-one in his right mind wants to be cut open and have bits and pieces removed . . . but when the alternative is death, he 'bites the bullet' and accepts the pain and suffering as being unavoidable and necessary. In the same way, solitary confinement is extremely unpleasant, but I see no alternative to it. Unless and until one can be found that's at least as successful in reducing prison violence and enforcing compliance with regulations, I fear solitary confinement must and will inevitably continue. That point appears to be lost on Mr. Bauer . . .