An employer has an obligation to a safe and drug-free work place for his employees. Marijuana legalization in Washington has not one thing to do with allowing stoners a job. If an employer decides on a zero tolerance policy, he has that responsibility and right, as do the other employees.
Marijuana is illegal in the feds' eyes. Until there are changes in the drug laws on the federal level, do not expect stoners to obtain a special privileged pass on their drug usage.
Employers and their employees have the right to a safe and drug-free workplace, and Washington's new marijuana law does not change that one bit.
It will just eliminate a few potheads from the employment rolls.