I have to add that it's not just the DM that rules what races are played in the campaign, but the players too. Vetoing by "Our PCs kill that race on sight" works.
You could say that but, you'd be wrong. If the DM rules that you can't play a 1/2 Dandelion 1/2 dragon, you don't ever get in the game to be killed.
Notice, the player never had a say in the rule...