There is actually a big difference if the casino has no restriction on withdrawals of someone which deposit/s is unconfirmed. Imagine if a player wins with their unconfirmed transaction, and they'd be able to freely withdraw said winnings. This player could also try to double spend the same UTXO with a much higher fee so it will likely confirm first making the previous transaction invalid.
You are correct and I agree with you that there are too many difference to the restriction of the withdrawal and no restriction of the withdrawal. But if they restrict withdraw before the confirmation then they would not be able to scam as they can not withdraw the fund and if they deposit later then the site will catch that and will suspend the gambler.