I don't think that All Casinos required kyc only for mistake or violate the rules. Sometimes money laundering is done by these gambling websites, I have come across many casinos that require KYC to avoid money laundering when withdrawing large amounts of funds. So All investors should read the rules carefully before investing.
They do that to know the person that has sent some huge funds into their casino without playing and then wants to withdraw. That's the cause of the sudden questioning and requirement of these casinos for KYC.
No casino today is kyc-free and all of them has to comply to the rule of the country that they're found.
It's part of the rule of the government that they want to secure the funds coming in to them and that's why when there's a sudden transaction that deposited and then eventually wants to cash out without any activity, that's where they flag a kyc to that user.