Most individuals put around constant violence or threat of such are permanently changed, an unpleasant/undesired change. So for may non-violent crimes many would consider prison a cruel punishment. At least with how US prisons are set up.
My understanding is that historically the "unusual" in "cruel and unusual" indicates unusual from everyday reasonable life. Constantly being around violence to an extent it changes the average individual permanently is unusual for every day reasonable life in the USA at least.