gamerprinter
Mapper/Publisher
Well, it depends. Unless I spend hours and hours describing the setting of Dark Sun to you, you might not realize just how much defiling is hated by the populace - until you cast a spell in the town square, and get chased out of town.
Likewise, if my campaign has a noble warrior sergeant-type race, unless I go out of my way describing how awesome these guys are to you before PC Gen, I'm kind of responsible if you decide to attack him right off the bat thinking "he's just like all the other mooks".
Well if a character was born in Dark Sun, he would be expected to know this - it all depends on whether the PCs came from some other setting and was suddenly teleported into Dark Sun, its possible they don't know not to cast defiling spells in public. However, if the PCs are supposed to be locals and the DM never mentioned the danger of defiling in public - that's a DM problem, and not a player problem.
The DM is not supposed to keep obvious knowledge away from the players.
Sometimes a setting has secrets that aren't obvious to everyone, and that by rights requires the PCs to discover the truth on their own.