Heh.
When I set up a game, I create a handout telling players exactly what is allowed. I am not the only person on this thread to do so.
I will note, however, that said handout also implies rather strongly that anything not listed is not allowed. Nor are there any reasons given for this, apart from the obvious "X (the allowed stuff) is how the world is" and, by implication, "Not-X is how the world is not, at least as far as you know at game present."
This last is important because, to me, "no drow PCs" does not imply "no drow in the game world".
Moreover, if there are no elf PCs listed in the background documentation, there may be a note that elves have all disappeared, or there may not be any knowledge of elves (either because they are hidden as part of the milieu's secrets, or because there are none).
The background document tells you what you know. "Why is X like Y?" is a question that is (much more often than not) best resolved by game play.
RC
Just imagine this being PbP....."No tieflings" is listed amongst the creation rules. Even if teiflings are not mentioned in the background, them it gives you the info up front.
That character creation ruleset is a very important one.
Which has more options for ME?
The game that says:
Elves, Humans, Orcs, Dwarves PCs only.
Or the game that says:
No tieflings
Both have no tieflings allowed, and both are clear as to what is allowed. Want drow or eladrin, you use the second game.
The why is always obvious. Either you find out during play as the game is meant to be, if not in the background of the world. OR the very fact that not all games are run the same way. The same DM can always run the same game format with races allowed, or may run different ones at different times. Each game is different is ALWAYS the why. That is the purpose of D&D.
Personally I want a list to read rather than a TL;DR paragraph of crap to tell me what is and isn't allowed. That is what lists are for.
If it doesn't say Yes, then No is assumed such as the first example. If it doesn't say No, then Yes is assumed like the "no teiflings" example.
1st example:
Player: Can I have an eladrin PC?
DM: Elves, Humans, Orcs, Dwarves PCs only.
(Player: Why can't I have an eladrin?)
The answer is already No.
2nd example:
Player: Can I have an eladrin PC?
DM: No tieflings.
(Player: Why can I have an eladrin?)
The answer is already Yes.
Why ask the question if you know the answer? Why ask the question when the answer doesn't matter?
See how silly the questions in parenthesis look?
aaaargh I came back to this thread...
Point being, character creation rules, like you say, don't really say anything about the world, that is to be learned through play, or through any pre-game info about the world the players need.