I strong disagree with this. The existence of things like slavery in now way incentivize players to commit those sorts of acts in game. Not any more than murder and kidnapping do with the existence of those things in a normal game. When it comes to things like Defiling and being a Templar, that's no different than being a blackguard/anti-paladin or death domain cleric in the base game.
Since the beginning it has been assumed in all settings that the PCs will be heroes, but also provided some small number of things for those who wanted to play evil. Dark Sun is not greater than normal at these things. I just offers different paths than the standard anti-paladin or cleric of an evil god. It may seem like it's incentivizing things more because you're used to the traditional paths and so those don't stand out as much.
Again I say the issue is not existence but focus.
I may have had bad luck and ran into terrible DMs. But having over half the DMs I played with, knew, run for players I known, or ran campaigns I been told about heavily focus on the iffy aspects of them is not just a coincidence.
DMs tend to go harder on a setting's themes and foci. They get looser or tighter with the theme's and foci's impact.
Forgotten Realms and Dark Sun both have slavery, cannibalism, forcred breeding, nature destruction, and transformation via horrible acts. Only DS focuses it on it. FR doesn't focused on the ant-paladin, bloodknights, and supremacist rangers. It has evil necromacers, demon cultists, death clerics, and martial raiders.
And that's the crux of the issue. The iffy parts of FR, GH, Strixhaven, or Wildemount are less problematic in today's world than the iffy parts of DS.
So WOTC, a corparation focused of its visiblity, doesn't want to touch DS in case they mess up and or it doesn't sell enough to be worth the effort.
And you are right. We are used to the problematic parts of Forgotten Realms and Dragonlance. That's why WOTC supports them. We wont care about it as much.
It's unfair toeDark Sun. But the point of Dark Sun was to be different and subvert the tropes we were used to.
I mean, we are now having discussion on the issues with Dark Sun after 30 years. FR gets this every 6 months.