Part of casinos' terms of service is banning your account if you cannot justify your source of income or they think it comes from an illegal source, so if a casino bans your account. You have 6 digits in your account; where will the money go, and do they have the right to keep the money? There is no provision; I read they will turn over the money to the authorities.
Is it legal for casinos to keep the money?
If it's on their terms and conditions, then it's legal if the casino keep the money if it comes from an illegal source, however I don't think it's right if the casino does this deliberately, especially when gamblers hit nice prizes on the games provided by the platform. This rule should be only taken into consideration in very rare situations where the source of the gambler's funds is blatantly from illegal sources, such as the ones connected to criminal organizations and theft. Then I would say it's right that the casino returns the money to authorities.
But if that is not the case, I see no reason why casinos should be requesting every average gamblers to provide their income sources in order to play or cashout funds from the platforms. It just mean extra bureaucracy and inconvenience for the players who might feel annoyed and tempted to leave the platform for this reason, going to another websites where they can have a simpler and more enjoyable experience.