I have nothing against disallowing something from a D&D table. I have done it, and will continue doing it for my campaigns. However, I do it for very different reasons than others in this thread do, it seems. I do it for balance issues and design problems. I don't allow Yuan-Ti Purebloods at my table, as they are way OP, I don't allow the Order of the Scribes due to how the subclass is designed, and I don't let people play Wild Magic Sorcerers at my table, due to them being capable of accidentally screwing up other characters, which could ruin the fun of the game.
I never disallow something because of the world. Never. D&D worlds are infinite and imaginative. There are always the core or base races of a world (like Humans, Elves, Dwarves, and Halflings in FR/Greyhawk/Dragonlance), but there are always outliers, and the characters are supposed to be outliers. It doesn't diminish the definition of my world if I allow a character to play a tiefling in a world where tieflings literally don't exist. It's their character. It's my world. If they want to play a strange/unique race, I'm not going to stop them. People will be constantly giving them strange looks, which is a side effect for playing a strange race. It doesn't hurt me or my world to let my players have fun.