I think if they know if their casino is illegal in their country, they must ready for something like that. In my country, gambling is prohibited and whoever play and caught, go in jail. I think they just need to follow the rules from the country.
Or the casino business owners can run their casino in a secret place, and only trusted people who can join with them, and they only invite people who they known before. Usually, this type of casino has wealthy members who don't want to get caught by the police, and they pay for some money to corrupt police to protect their business. I think that it has run in many countries in which gambling is prohibited, but they are free to operate the casino.
Lol thats impossible to happen,because casino owners are pointed already being a gamblers advocate so they will be on watch list and each time surely they will be visited
And besides another thing that there will be a asset who will tell the police about the operations .in other word Casinos cant operate that well unless they will conduct Guerrilla type of gambling houses that can transfer place from time to time and will only have rich players that can afford that style of gaming