Some countries collonised others: crime of ancestor
But those countries used slaves and stole resources, making those countries wealthier. That wealth allowed them to develop better technologies, making them even wealthier.
So the argument is that while the original crime is not the responsibility of those alive today, the proceeds of crime should not be kept - they should be returned. In this case the proceeds are wealth, so a monetary reparation is appropriate.
Is my train of thought right? Because it seems to make sense to me.