Dickens was focused on prison reform
In the Victorian era, prisons were seen as a humane way to sentence offenders in contrast to the capital and corporal punishments often used at the time. However, Dickens observed how it was still largely unjust and could harm offenders further, thus becoming one of the first advocates for prison reform.
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Solitary confinement: a form of imprisonment distinguished by living in single cells with little or no meaningful contact to other inmates, strict measures to control contraband, and the use of additional security measures and equipment. 
Dickens visited America's Eastern Penitentary in 1842 and was horrified by the conditions of solitary confinement imposed on many inmates; he believed that "very few men are capable of estimating the immense amount of torture and agony that this dreadful punishment, prolonged for years, inflicts upon the sufferers" , and was worried at what implications such punishments would have in England. At this time prisons were considered a humane and even lenient alternative to typical sentences of capital punishment, transportation abroad or hard labour, and were part of gradual movements by Victorian reformers to make the justice system fairer, particularly for working-class people who were often penalised unfairly. However, they were still damaging and harsh, with terrible conditions which many legal reformers were willing to overlook because prison was better than hanging, and for Dickens to speak out against them was surprisingly progressive, indicating a genuine concern for the welfare of the poor which went beyond the performative moral reforms of many supposed progressives of his day. His description of solitary confinement in his American Notes called attention and scrutiny to a previously ignored issue and opened up a more genuine debate on improving conditions for prisoners by giving them useful work and educational opportunities in a significant milestone for the Victorian legal system.
Dickens was not the only prison reformer of this period: Elizabeth Fry (1780-1845) had worked throughout her life to improve conditions in Victorian prisons, offering humane work and support for women and children, so to characterise Dickens as uniquely progressive in this way is wrong. Additionally, his writing was on an American prison, meaning he could safely distance himself from criticism regarding the issue of prisons in England.
Dickens advocated for more humane prison conditions, including an end to solitary confinement, even though many justice campaigners at the time accepted prison as more humane than the death penalty and therefore not in need of reform. His writing called public attention to how inhumane prisons could be, opening up a wider debate on the function of a prison.
Rejecting the premises
Dickens was not the only prominent prison reformer of his era. He focused on an American prison in his writings, meaning he could distance himself from reforms which still needed to be made in English prisons.