Sharecropping was abhorrent but was distinct from slavery.
Slaves may be killed legally by their owner.
As for conscription, it's a similar situation. The constitution allows the government to "raise and support Armies" under article I section 8. This has been interpreted to mean the usage of the resources of the nation to build an army as approved by Congress. It can be money (taxes), land (eminent domain), or people (conscription). Again, because of historical precedent set in statutory and common law, as well as being included in the constitution, it has not been deemed to violate the 13th amendment and instead exists as another exemption.
At least, that's my understanding.
Conscription stems from the understanding of the militia as being universal and the express power of Congress to "To provide for calling forth the militia to execute the laws of the union, suppress insurrections and repel invasions", not from the power of Congress to "raise and support Armies".
Thus, controlling your own wealth (right to property) is a basic human right.
Dignity is not a human right, since it is subjective.