Its simple.
Its to prevent a double spend attack, You could simply send money and withdraw it right away. and then the confirmation fails.
i see..... but you are wrong , you know why ? cause when you deposit they ask you FIRST to wait for confirmations than u can play , so think again

when you say instantly in the thread title it means 0 confirmation needed, if it needs atleast 1 confirmation then the deposit isn't instant like what you said