A simple suspicion is enough and the owner of the funds will have to provide a huge amount of supporting documents (to prove that he is not a criminal). Online casinos can also be understood. If it turns out that criminals have cashed out funds (obtained by criminal means) through casinos in the past, then law enforcement officers will make claims against the casino. They will demand that the criminals be identified. This is why online casinos introduce KYC procedures.
Unfortunately, in our time, any casino can require any user to undergo the KYC procedure. This may be due to a request from law enforcement agencies or other circumstances.
Not all casino, but they should be licensed at least. Before requiring their players to undergo kyc. Because if the site is not licensed and anonymous, do you think it is fair for their players to comply with KYC protocol? I don't think so.
So the casino should make sure they are operating legit and with necessary license before restricting their players owed to KYC docs.