Suppose that you and I make a contract that, in exchange for you giving me a badly-needed $20 today, I will mow your lawn tomorrow. Then suppose that after I spend the money, I change my mind about the wisdom of this deal. Can I repudiate the contract on the grounds that my labor for you is no longer voluntary? If so, then people will have no incentive to provide payment for services in advance, and it will hinder the economy in many cases.
Why do we need a rule against people selling themselves into slavery? When has that ever been a common practice? If someone is dumb enough to do so, don't they deserve the consequences? Those people will, like many other losers, weed themselves (and their foolish tendencies) out of society in a Darwinian manner, and it will cease to be a problem. But in order for that to happen, you have to let the market be free.