> Forced prison labor is very clearly not chattel slavery.
But it is, if unpaid, slavery.
> And I'm not a constitutional lawyer, but it seems to me that "except as a punishment for crime whereof the party shall have been duly convicted" qualifies "involuntary servitude", not "slavery". And in any case, lawyers like to err on the side of ruling out any possible misinterpretation, no matter how ridiculous.
No. The distinction between the two is that involuntary servitude covers being forced to work while being paid or otherwise compensated; a slave, in contrast, doesn't get paid. Both are forced labor, but one is slavery. US prison labor may be one or the other, depending on payment.
(An English teacher would also sigh at you for your parsing of the clauses, I suspect. Your interpretation would require a different arrangement of the commas, i.e. Neither slavery COMMA nor involuntary servitude except as a punishment for crime whereof the party shall have been duly convicted COMMA shall exist within the United States...)
> If one was with a child, and the child saw a chain gang working, and the child asked "Are they slaves?", no sane person would say "yes" and leave it at that.
Correct, because at a glance, you don't know if it's forced labor. Some states do voluntary work gangs; in some scenarios it's even considered a perk.