Why exactly is slavery wrong?

This is the provocative title of an essay posted on the "Daily Bell" website:
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It begins:
"Hardly anyone in America disputes that slavery is wrong. It's an institution that must be prohibited, never to be readmitted into society. Indeed, slavery is so wrong that it is very tempting for decent men and women to devote time and resources to ferret it out abroad, wherever it is still being practice in measures large or small.
"But why is slavery so wrong, so horrible? It is, simply put, because it obliterates an individual's self-governance or sovereignty. It takes over that person's life by others who have not obtained any authority to do this. Of course, one can have peaceful relationships with others that could superficially appear like slavery does − when one is employed by another, virtually night and day; the difference is, however, that employment is voluntary. It avoids subjugating another against his or her will but involves coming to mutually agreed to terms. Sure, at times one party might be hard up a good bit and will more easily yield to the terms another wants but still, the employment relationship even at its most unpleasant contains the exit option. You are free to exit it and no one may go capture and return you to the employer. You are free, even if you may go hungry for a while.
"Slavery, in contrast, means you are not free even if you're terribly well fed, even if your health is well taken care of by your master, even if you are being educated at your master's expense, even if your retirement is guaranteed. If the condition of your existence involves being coerced to perform services for someone, if you may not leave without loss of everything, if you are obligated to others regardless of your refusal to give your consent, you then are enslaved or, at best, suffer the condition of severe involuntary servitude."

It's well worth reading the whole essay, PLUS the comments that follow it. The writer's theme is that collectivism is slavery - at least for people who are trapped in a community without the freedom (for one reason or another) to leave it. I'm interested to see what IN members think of the topic..