I confess I'm not too familiar with Dark Sun, but I thought a couple of the major city-states were ruled by Mage-Kings or something like that. Obviously I could be wrong, and would greatly appreciate any corrections.
Sorcerer-kings. (Probably should be wizard kings since 3.x.) They're all powerful wizard/psions.
On Athas, arcane magic relies on plant energy for use. There are two ways of casting arcane spells. The default is defiling (in flavor terms; in rules terms the default is preserving. Confused yet?).
Defiling simply drains all plant life energy within a certain radius to power a spell. It's hideously wasteful. A lot of wizards (or I guess now warlocks) can't or won't be responsible in their use of spell energy. Defiling doesn't just kill plants, it also ruins the land, rendering it barren for years to millennia. Just one reason Athas is a horrible wasteland. Also the main reason why everyone hates wizards.
Defiling magic causes pain to nearby creatures; in 2e this was codified as an initiative penalty to
anyone within the defiling radius.
All sorcerer kings are defilers (at least those that aren't plot spoilers, heh). Because there's only so much plant life to go around, they still hate other defilers, if only as competition (and also because they might understand the sorcerer king's spells; can't have that!). As a result, the laws of the cities still ban arcane magic (except Tyr, where only defiling is banned), but some sorcerer kings do allow a small number of defiler servants.
Preserving is a much more environmentally-favorable way of casting spells. It only takes what energy is needed and doesn't ruin the land. Unfortunately, most people on Athas are illiterate and ignorant, and can't tell the difference. If they think someone is a wizard, they assume they're a defiler, and wouldn't even know the word preserver. Of course, preserving is more subtle; it's easier to pretend you're using psionics or are an elemental priest or some such, which is one reason the Veiled Alliance is still alive today.
In 2e, defilers gained levels faster than preservers, and could
not use preserving magic. (This rule was often broken in the novels, where at least some defilers simply
preferred to defile, presumably due to the nearly addictive rush of excess magic.) Whereas preservers could
occasionally defile, and doing that too often made them into a defiler permanently. (The wizard Sadira from the novels, a preserver, used defiling almost obnoxiously often, and
did get called on it.)
In 4e, however, preserving is the default, at least for PCs. A wizard or warlock can
choose to defile, at the cost of harming their allies, in order to reroll a spell's attack or damage roll.
(NPCs often simply inflict damage on everyone nearby, or just on enemies in a certain radius, and sometimes gain a benefit from doing so. The nightmare beast is actually nastier when it misses, since it only defiles when it misses and it does horrid damage with defiling, even before it dishes out continuous lightning and necrotic damage with its spell-like abilities.)
Furthermore defiling isn't quite so harmful, in that defiled land can still generate spell energy; it's basically flavor text. (Of course, defile an area enough times and it still becomes the classic dead arcane magic zone.)
