They call it "involuntary servitude." I read it as:
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 ...
It's a distinction without a difference. Even the Wikipedia article explicitly says that involuntary servitude and involuntary slavery are used interchangeably. [0] I feel you're getting caught up in word choice instead of arguing the actual implementation and interpretation of the law.
