Just because it is in the constitution doesn't make it right. That is a document designed to be changed, after all.

Women couldn't vote for quite some time. Some people weren't fully human. Only landowners could vote. And so on.

Furthermore, it being in the constitution still doesn't detract from it being slavery.

Also, the Constitution is a minimum bound on rights, not a maximum. We don't need to change the Constitution to make just laws.

We were comfortable calling slavery by what it was back then, and we shouldn't shy away from the term if that's what's still happening. The constitution defining slavery doesn't make it right. But it does imply we shouldn't mince words.

Even if we accept that it being in the Constitution makes it right, it is permitting slavery as a punishment and not as an expedient.

i agree that's one way to read it, but it seems plausible to me that the intent really was just as a winnable end-run around abolition.