To be clear, defilers can cast spells in settlements and around NPCs, if they're prepared to face the consequences; the thing to remember is that these consequences also exist for preservers.
This is not true. Defiling is far more hated than preserving is. Consequences will hit the defiler far more often and generally to a greater degree than preservers. And you keep ignoring the fact that preservers can still sneak spellcasting in town where the defiler can't because he can't hide the pain and effect on life. Both have it hard. Defilers have it harder.
"In most cases, defilers are outlaws (even in the eyes of the corrupt sorcerer-kings), so they keep their magical abilities under cover. Unlike preservers who have a loose organization in their underground, outlaw defilers tend to be loners, keeping their ambitions and powers to themselves."
There is no such language under the preserver class.
"...druids tend to be very apprehensive about wizards who venture into their realms, as they might turn out to be defilers."
Defilers, not preservers. Druids also get bonus xp for defeating defilers, but not preservers.
The fiction and in-game text makes it clear that most ordinary characters recognize little-to-no difference between defilers and preservers (especially since preservers are presented as having the option to defile anyway);
Preservers have no option to defile. They literally cannot do it. Both the mechanics and the lore below support that.
"All wizards must
decide at the beginning of their careers whether they are trying to work with nature or without regard for it. In Dark Sun, this means a wizard must be either a defiler or a preserver."
Once you make the choice, you are done with the other type of magic. That is the only lore about wizards having a choice that is in the boxed setting. You have to make it when you learn magic and are stuck with your choice from level 1 onward.
even then, many defilers are presumed to operate at the behest of the Sorcerer-Kings, whereas preservers are typically marked as outlaws and enemies of those same rulers. So in that regard, defilers are no more disincentivized to play than preservers.
This is actually false. They are all outlaws except those few who work for the sorcerer kings.
"Defilers
Cities are the hardest places for inexperienced Defilers to survive. Not only are they hunted by the agents of the monarch, but few citizens will provide them with refuge or assistance-in addition to fearing the king's wrath, most people resent what the Defiler's magic does to the land. For this reason, most Defilers lead solitary lives of constant apprehension, associating only with the shady elves who sell them spell components."
Remember, preservers still took life energy from nearby plants; they just didn't kill all of them and scorch the soil.
They didn't kill any of them. This is from the class itself.
"The preserver is a wizard of the old, established school of magic. In the give and take of spell casting, preservers have mastered the balance. A preserver's magical spells are cast in harmony with nature.
When a preserver casts a spell, there is no damage to the nearby environment."