EzekielRaiden
Follower of the Way
Because that was literally the justification given for opposing player agency in character creation. That it is disruptive for players to choose anything but the predefined choices granted to them by the DM, and specifically because it is disruptive, DMs are justified in forbidding it. (Meanwhile, it is totally unacceptable for DMs to forbid disruptive character behaviors from their players, no matter what those behaviors might be nor the degree of their disruption, with no explanation for this discrepancy other than "Because the DM says".)No one. Why would you think someone who played a tiefling or Dragonborn was “not nice”? Genuine question, I really don’t understand your logic.
If we assume all of the players are well-behaved, then why is the DM forbidding a well-meaning, undisruptive request? The only reason to do so is if the DM is crapping on the player's request, which (per your instruction) I have assumed is non-disruptive because the players in question are being presumed to be nice.