It's really undermining the atrocities of the transatlantic slave trade by comparing it to prison. We wouldn't compare it to indentured servitude, which is much closer to the penal system (monetary debt vs social debt, but both are contacts even if not purely voluntary). The federal government also doesn't have complete ownership over prisoners. Yes, prisoners are mistreated, but what they face isn't at the level of those from the slave trade and so you're effectively diminishing those atrocities.