According to the original campaign setting, page 20 under the sub-heading Wizard Classes:
The defiler is a wizard who activates tremendous magical energy without regard to its effect on the environment. With the casting of each spell, a defiler destroys a portion of the world’s ecosystem, rendering it dead and sterile. The means by which a defiler learns and uses magic is comparatively easy to master, so he advances quickly. A defiler can be either a noble, freeman, or slave.
The preserver attempts to use magic in concert with the environment. Learning how to wield such magic on Athas is especially difficult, so the preserver’s advancement is slow. A preserver can be either a freeman or slave.
To summarize: a character of any social class may be a defiler, but only freemen or slaves may be preservers. Nobles, as rentseekers, cannot help but engage in the same exploitative modality in relation to the environment that they do with their servants and slaves.
The illusionist is a specialist wizard who wields magical illusions. An illusionist can be either a preserver or a defiler, and will advance in levels accordingly. An illusionist can be from any social class.
A noble can be a preserver, but only if he is an illusionist.
“Come, friend. Accept your reward for the successful completion of your assignment”, said the nobleman as he gestured towards a trunk full of gold coins.
Arik the former mul slave ran his hands through the gold coins greedily, fantasizing about the palace he will soon be living in, full of servants and slaves. Suddenly the trunk of gold disappeared, and the gold coins had transformed into iron shackles.
“Time to learn your place again, slave”, said the nobleman illusionist.
Then you have a dramatically different opinion of 2e game design than I do, which is, IMO, wildly at odds with racial level limits and class/alignment restrictions.
Perhaps the designers wanted to prevent ‘Mary Sue’ PCs: Goodly perserver noble PCs who have everything, want for nothing, and can change Athas from inside the system - that’s the antithesis of nearly every DS trope.
I would describe the situation this way (Of course this might not apply everywhere):
Nobles find it much hard to hide their practicing of magic because of increased scrutiny and being surrounded by servants, even interaction with many templars perhaps even their sorcerer-king directly. The veiled alliance tends to distrust nobles because of their involvement in the oppressive system and fears being exposed, so while they may accept the aid of nobles and their patronage from time to time, as a rule they refuse to tutor them in the ways of magic. All of this leaves a situation in which it is both easier and more likely for a noble to gain the tutelege and trust of a Sorcerer-Monarch then most anyone else in Athasian society especially as it helps to further cement/extend the Sorcerer-Monarch power to have an already wealthy and connected member of society among his defilers. While at the same time making it nearly impossible for a noble to learn the ways of a preserver.
It would have been a lot better if they had included a paragraph like this instead of just stating what they did, with no elaboration.