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.
As I know, the casino does everything possible so that users do not earn a lot on welcome bonuses and do not abuse them. Bonuses are more like part of an advertisement.