Slavery is still LEGAL in the United States

You didn't think so?  Neither did I.  I thought the 13th Amendment abolished slavery.  But I hadn't paid attention to the conditional clause:

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. 

Here's some discussion of the penal exemption:

The Thirteenth Amendment exempts penal labor from its prohibition of forced labor. This allows prisoners who have been convicted of crimes (not those merely awaiting trial) to be required to perform labor or else face punishment while in custody.

It was apparently considered noncontroversial at the time, or at least legislators gave it little thought... slave labor as a just punishment for robbery, so that the thief's labor could be used to pay recompense to their victims and to society...

Various commentators have accused states of abusing this provision to re-establish systems similar to slavery, or of otherwise exploiting such labor in a manner unfair to local labor...prison labor continues in America under a variety of justifications. Prison labor programs vary widely; some are uncompensated prison maintenance tasks, some are for local government maintenance tasks, some are for local businesses, and others are closer to internships. Modern rationales for prison labor programs often include reduction of recidivism and re-acclimation to society.

Blogged because the principles involved here have been cited in discussions of an ongoing nationwide labor strike by U.S. prison inmates.

In addition to loss of life, the strikers, led by a network of incarcerated activists who call themselves Jailhouse Lawyers Speak, have put out a set of 10 demands to overhaul America’s creaking penal system. High up on the list is an end to forced or underpaid labor that the protesters call a form of modern slavery.

Addendum:
A followup article on the strike and some data:

More than 800,000 prisoners are put to work each day cleaning, cooking, farming and mowing, in some states compulsorily. In states like Louisiana compensation is as low as 4¢ an hour, even though prisons are entirely reliant on such labor.