No, the law does not require disciplinary action for drug use or, indeed, any on-worksite crime. It can be foolish for the employer to not take action, since if they are aware of criminal or antisocial behavior or substance abuse which could lead to harm or injury but nonetheless fail to take action to protect their other employees, they could be liable for any injuries, thefts, etc.; but if they are willing to take that risk, that is their choice, and the law lets them do this.