The Washington Supreme Court found that the state law governing issuance of permits is not preempted by federal firearm laws laws, under which drug users cannot possess firearms.
The majority opinion from the Washington Supreme Court is available here, and disagrees with that central point of preemption (kind of);
We note that the Washington State Human Rights Commission, the agency charged with investigating employee discrimination claims, acknowledges that “it would not be a reasonable accommodation of a disability for an employer to violate federal law, or allow an employee to violate federal law, by employing a person who uses medical marijuana.” Laura Lindstrand, Wash. State Human Rights Comm’n, Washington Non-discrimination Laws and the Use of Medical Marijuana at 1 (June 7, 2011), available at http://www.hum.wa.gov/Documents/Guidance/medical%20marijuana.doc. Though an employee is still free to sue an employer for wrongful discharge, the commission will not investigate claims of discrimination due to medical marijuana use because federal law prohibits marijuana possession. Id. at 2.
In other word, the federal law preempted the state law. These issues are not directly on point, but pretty similar.
Interesting how these border states arrived at diverging conclusions.