If one truly believes in the free market system then so should they support unpaid labour (slavery). If both parties consent to the exchange of labour for food and lodging then under a truly free market, that should be allowed. By interfering with that trade through government regulations (socialism) or the like, one is stifling the natural balancing effect of the theoretical "invisible hand". To put it bluntly, you either support slavery in a market system or you support the abolition of slavery through socialist means.
Science • Technology • Equality