They could just tell the truth about it that they are charging withdrawal fee,
I don't think they ever hid that they are charging withdrawal fees. in fact, they are saying that they are charging withdrawal fees. you can check their ToS and you'll find what you are looking for on "Withdrawals". you can even see when trying to withdraw how much they will charge you for the withdrawal fee.
Well the case here has a different view and scenarios for different forks, as we all know that every casino with their roles as some sites give the customers the choice to chose the fee and transaction time if a casino charge an extra fee aside from the blockchain fees it may become a double burden on the customers most especially those who want to withdraw a small amount but those withdrawing big amount may not feel the impact of the charges but a regular player who want to withdraw small amount.