Certain transactions cost more to withdraw, such as transactions that spend more than one input. Those transactions are unavoidable for a casino, which will almost certainly have more deposit transactions than withdraw transactions. If they chose to charge withdraws a fee based on what was actually paid, it introduces a luck based component around how many inputs your transaction will have - you might pay $1.69, but you could also pay $6 if there were many inputs used. I don't think anyone would be happy with gambling on fees, so the easiest way for sites to handle it is to charge a fixed fee that covers the average withdraw fee.