because you will have access to the coins sent to your exchange deposit address. so for example, you deposited 1btc to your poloniex account, 1btc will be credited to you and the 1btc on your deposit address can be accessed by you and use it to send to another address. basically it is stealing
Yes and no. They can perform checks before investing money in exchanges or loans I think. So if the money was spent, they can disable the transfer.