Legally speaking, Casino is not allowed to confiscate the deposit of the user. They are only allowed to confiscate the profit but the deposit should be refunded. The ToS of the casino will not govern over the law of the country which they are under jurisdiction.
Unless the source of funds is proven from illegal such as money laundering. That’s the only time casino is allowed to confiscate even the deposit.
You're right. Many casinos scam people by hiding behind their ToS. You can't make lawful agreement with someone, even if that person accepts and signs everything, if that violates the local law. For instance, I could sign an agreement that after my death my body is to be buried under a tree in my yard, but if the local law forbids dead people to be buried outside of the designated cementery, there's nothing you will be able to do about it. The police will enforce the law and your body will be moved from your house, no matter what your last will says.
If the law does not allow them to take your money, the fact they put it in their ToS doesn't change anything. If they do it, they can be sued.