Over-imprisonment isn't the reason, fear of death is. The fact that the US prison system is rampant doesn't really make the living condition of one given inmate unusually cruel. The high risk of them catching a deadly disease does.
I think what GP is saying is that Kim’s probable treatment in the US criminal justice system would not be unusual, it would in fact be the usual treatment for US inmates. Inmates in the US regularly fear for and lose their lives, pandemic or no pandemic.