For those who say that this is improper or wrong: not really. It is not a full sentence, true, but thats because these are both exclamatory sentences (like "Who?" or "Stop!"). No, it does not give a reason, but in my experience this is rather a common sentence to reinforce an order. Think of it as a shorter form of "Why do you need to do this? Because I told you to/because I said so!"
Wouldn't one say "deswegen" or "darum" then?!? I have never heard someone say "weil" like it is done here.
I agree - "just because" explains most German grammatical questions. The others are explained by "Deutsch ist verrückt"
I take issue with the answer "Weil!" I've never heard a native speaker respond to the question, "Warum?" with, "Weil!" The correct response is "Darum!" Schluss! Ende! Basta!