In any case, before registering at the casino, you need to read the terms of use to know what you can do and what is forbidden to do at the casino.
Almost all casinos have the same terms of use. The same things are prohibited everywhere, such as the creation of multiaccounts and all sorts of frauds and manipulations with the account.
The players want to earn more, so they use all sorts of manipulations. In casinos, people are willing to take risks, including the risk of violating the rules of the casino.
Abusers is not taking risk at all because they are just abusing the bonus and hope that they will not caught. In the event that they are caught, There original funds will be refund by the casino and just ban there account. They are just risking there KYC since its a requirements to recover there funds but since theres many people especially on 3rd world country that willing to sell there KYC details in exchange for a penny. This scammer/abuser keeps coming back since they have unlimited source of ID for KYC verification.
This is the risk on the casino side by providing bonus to all new registered player. This is the reason too why KYC requirements is starting to become a mandatory on some casino.