Part of this may be down to the longevity of the campaign world and its relationship to the players. ... Other GMs however have a single long-established campaign world that may have existed for decades. Their current group of players may be relative newcomers to this world.
Well, just because someone needs to ask themselves if they're
really justified in (saying no/nerfing something/attacking PCs) doesn't necessarily mean that the answer is "no".
I mean, I'm a pretty permissive DM when it comes down to it, but when I run
Dark Sun, I expect my players to bring
Dark Sun characters-- no Gnomes, no Paladins, and if you cast arcane magic in public, you better
pray the lynch mob gets you first. (And yes, I'm still salty about that sidebar in 4e.)
And the thing is? None of the established D&D campaign settings were written the way people are trying to run them--
The Complete Book of Humanoids for AD&D 2e was
written for them, random tables in the AD&D 1e DMG had encounters with humanoids
in metropolitan areas, and while it's clear they're not even second-class citizens and don't enjoy legal protections, their existence is at least
conditionally tolerated.
This is canon. Dungeon Masters who run it differently are changing it for their own purposes.
Homebrew is a different story, of course. Homebrew is homebrew.
But then, if the Dungeon Master is the author of the setting then it is the DM who decided that humanoids would be treated this way by "civilized" folk-- I will
guarantee you they made this decision 30 seconds after seeing the player's character sheet-- and thus it goes back to the fact that they designed their homebrew world this way, and the question: why?
Hence my post. DMs have all of the authorship and the authority in their worlds, but the title of this thread and many of the comments within it are worded to shirk the responsibility for it onto their players.
And, seriously, what is with all this passive-aggressive nonsense? If you only want certain races in your game, make a list; when players ask to play something else, refer them back to the list. Allowing the player to bring an unwanted character into your game and then punishing them for it is petty and childish.