The 13th Amendment to the US Constitution is short and sweet:
Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
Section 2. Congress shall have power to enforce this article by appropriate legislation.
Now, there are a few ways to interpret this guy. The most obvious is that human beings may not be considered chattel: trucked, bartered nor traded. The other bit, the involuntary servitude, this is a bit harder to pin down just as easily. Involuntary service is performed under duress or threat; it is service performed when trade would not be conducted voluntarily otherwise. One might easily make the case that mandating the purchase of services is involuntary servitude, being that one side of the transaction is involuntary. You might think of this as slavery on its head: where the master is forced by fiat to accept the service of others.
If Congress passed a law mandating that every citizen purchase ten gallons of Boudreaux's Butt-Paste every year, would it be reasonable to cry foul by the 13th Amendment? It would be involuntary service to Blairex Laboratories, Inc. on the part of every citizen of the US.
On the other side of these purported rights, let's imagine I was a window washer and the City of Alexandria decided that clean windows were a natural right. They then passed Clean Window legislation and insisted that anyone who didn't patronize my services would be subject to fines and/or imprisonment. What then, if I were the only window washer in town, would occur if I decided to pursue my dream of shucking oysters in Oregon. I would then be violating the natural rights of my former customers and would be subject to terrible penalties. I would be an immoral monster. Anyone who didn't step up and wash windows would be just as guilty. Indeed, in a world that claims we have the natural right to the labor of others must necessarily be one in which everyone becomes a supplier of that service. I will work on a mathematical model to prove this. My point is that mandating access to labor necessarily implies involuntary servitude.
Maybe I'll just swap my windows out for plywood.