What a weird stance. The slavers are the bad guys. You are supposed to stab them in the face. That feels almost shockingly non-controversial.
I have a hard time imagining what else about Dark Sun might be an issue.
As a setting about the perils and consequences of climate change, it feels extremely modern.
It's been a while since I've read Dark Sun, so it could be I'm miss-remembering, but... I think it's because the slavery is institutionalized, which means that most people in-setting feel it's at least a little
OK. If you stab the slavers in the face, they can call the guards and you're
the one who's breaking the law. It's not like in normal D&D, where slavery is normally considered bad by almost everyone except for a few outliers (or slave-taking races, in the older days). In Dark Sun, slavery considered normal.
And there's a slave race (mul) which not only is regular chattel slavery, but sex slavery of their mothers as well.
Plus, well, I just did a search on "dark sun slavery" and the first hit I got was a reddit post from the Dark Sun sub for someone whose PC bought a slave, and how to DM that slave, even going so far as to ask if the slave would start to feel brotherly love for the PC that owed him. So they probably would have a hard time of figuring out how to include slavery without also including an official way of PCs become slavers. At least now, if a PC wants to buy a slave, the DM is the one that has to do the work for pricing and legality.
And you know
if WotC said that the entirety of Athas decided to make slavery illegal, or even that muls were a species of their own and not a race bred to be slaves, that would anger a lot
of fans who would see it as "woke" or some other nonsense like that. I saw people being angry that they changed "curse" to "torment" for Ravenloft's Dark Lords, despite that torment is a more active concept and a lot of the Dark Lord's curses were very passive in nature.