The world is designed to the basic default PC races. I only ban outright, races that are too powerful. If a player wants to play something else and they know the possible consequences why ban it? You make no sense.
If a race or class they want to play is so contrary to normal play in the campaign you are running as to not be able to meaningfully participate in the story that everyone else is trying to play, that choice becomes non-viable.
"You can, but then you'd die" is, to many people, synonymous with "No, you really can't." To others, they think you are exaggerating, as just dieing instantly when you get into a common situation isn't fun and they don't believe you mean exactly what you said.