> "Don't be". What a silly way of wishing something not to be. It is a contraction of "do not be", which implies that the opposite is "do be", as if being were something one does.
> Previously I thought of this as natural English but now it's struck me as odd. Why can't it be "be not" or "ben't" or something?

Sometimes in English we use "be" as in "Don't be like that." When I was in high school a friend of mine used to say that to his girl friend. She had a bad habit of not answering a question. She would just clam up and not say a word. So she was "being" silent or stubborn and he didn't want her to be like that. But I agree that there must be a better way to express that thought. How about "To be or not to be?"
Howard
PS - Oops. I just noticed that I used some good old American slang. But I'll bet people all over the world say "clam up."
H