You get it!
Its far more interesting to look at a class like paladin, warlock or sorcerer and then say "how does this fit in this world?" rather than discard it entirely. Not everything will fit in every world, but that doesn't mean they shouldn't try. If a "purist" DM doesn't want clerics, tieflings, or such in Dark Sun, they can say "no". But its a lot easier to get rid of something than it is to try and put something back into the world. A hypothetical Dark Sun setting should try to get nearly all the classes and as many races as it can (that make sense; I don't advocate for tritons on Athas) but much like the rules itself, its easier to have a default "on switch" that DMs can turn off than have everything turned off by default and force the DM to do the hard work of figuring out how it fits.
This is true of any setting really; the first response to "how does X fit" shouldn't be "it doesn't, ban it", that should be the LAST resort.
And again, I don't mean Dark Sun should use the FR/Generic inspired looks and "fluff", even some mechanical differences (such as subraces to represent the Athasian variants) is fine. I don't need orcs in Dragonlance either, but I want to feel I'm playing a D&D game with some variance, not a new game. I want to explore what a Paladin (a champion who has sworn an oath and recv'd power from it) is in Dark Sun, not be told outright it has no place.
WotC's job it to show me how to say "yes", since I already can say "no" on my own.