The analogy to theft has genuine moral force. Enslaved labour was coerced. The people who performed it received no compensation. The wealth it produced went elsewhere. If we accept that forcing someone to work without pay is wrong, which the entire world now does, then the people who were forced to work were wronged and the people who benefited from their forced work were unjustly enriched.
The claim also draws strength from the clarity of the wrong involved. Unlike some reparations arguments that depend on contested historical interpretations, the basic fact that enslaved people were not paid for their labour is not disputed. It is the definitional feature of chattel slavery.
The compound interest argument has a certain logical consistency. If a debt is never paid, it does not cease to exist simply because time passes. The claim that the debt has grown rather than diminished is at least internally coherent, even if the specific calculations are contested.
