I think that
@Maddog is saying that forcing somebody to work to repay their debt (restitution) can be seen as a form of slavery. If viewing slavery as 100% evil, you can no longer force someone to repay their debts. This goes a way to explain why punishments for criminals revolve around "time out" (jail) and sometimes capital punishment, but less restitution.
We obviously haven't gone completely down this track as we still have sentences of community service as a form of punishment, which is a form of slavery - but it is slavery to the community as a whole, just as compulsory military service is slavery to the community. We don't mind slavery to the community. What has been made unthinkable is slavery to an individual - and that means no restitution. A criminal can be expected to "pay their debt to society", whatever that means, but not be expected to pay their debt to their victim.