You know that movie that says all crime is theft? At the crux of the matter? Like thievery is wrong because you are stealing an item from a person who owns it. Killing someone is wrong because you are stealing their life from them. The act of physical violence is wrong because you are stealing, let's say, someone's well-being. And so on. In essence all crimes are crimes because you are taking something from someone, something that they own and that they are entitled (?) to.

Not to say that thief is really the root of all crimes - I am not an absolutionist - but in the case of slavery, I would say that at a minimum slavery is wrong because you are stealing someone's time and what they do with that time. You are taking the fruits of someone's labor from them and not providing them with anything in return. In the case of indentured servitude, often something was offered in return (commonly in colonial America, a trip to America in return for say, 7-10 years of service, or what-have-you) but what was offered was not really worth the sacrifice that is made. Moreover in indentured servitude and also in slavery, you are bound in perpetum (made-up word) to do whatever your owner/master commands. When you are paid or employed you are given both the free will, and the means, to refuse orders. Because if you are a slave you have neither the free will nor the means (the money, at a minimum; often if we are talking about Americn, 1861 before-and-after slavery also consider things like education, etc) to walk away from your job. As a slave or an indentured servant you did not have the choice to quit your "occupation" : your occupation was your life, if you were able to run away you were both a fugitive AND destitute.

Moreover if you were caught, either as a true run-away slave or someone who looked like a slave (even if you never had been) you could be returned to slavery at any time.

Slavery is a problem because you have no choice and you are not able to make any reasonable choices. It is a robbery of your basic liberty and independence.

Moreover that's just pretending that slavery exists in a kind of vacuum where things like physical violence, sexual abuse/rape, and vindictive owners/awful tasks didn't exist. I have attempted to leave those out of the equation because while slavery opens the door for such atrocities you can always say they don't always have to happen as a result. I think that slavery makes those things kind of almost a given. If you have enough people that think that they own other people you are going to have that kind of behavior.

In the case of unpaid internships I think they are closer to indentured servitude than true slavery. Something is ostensibly given as a result of the labor; however the value of it is too small. The internships are seen as being "required" in order to gain experience since "one can't get a job without." Or "college grads are required to take a certain number of hours in internships to earn credits." (These are not in quotes because I am skeptical; they are in quotes because I believe they are the common given reasons.) The experience is seen as necessary. however in return for it the interns have no choice in what they are going to do, are often looked down upon by everyone else, forced to do ridiculous amounts of work that paid coworkers aren't (because they can just foist them off on others) and the interns are generally seen to be a lower, second-class "worker".

Them's mah sense.