From the opinion:
Under Washington law, there is a strong presumption against finding that state law preempts local ordinances. Although Initiative 502 establishes a licensing and regulatory system for marijuana producers, processors, and retailers in Washington State, it includes no clear indication that it was intended to preempt local authority to regulate such businesses. We therefore conclude that I-502 left in place the normal powers of local governments to regulate within their jurisdictions. [...]
Local governments have broad authority to regulation within their jurisdictions, and nothing in I-502 limits that authority with respect to licensed marijuana businesses.
At issue was the fact that Initiative 502 did not explicitly state whether or not local government would have the power to ban marijuana business. Ferguson interpreted this to mean the law doesn’t prohibit the local governments for creating bans. By comparison, Colorado’s marijuana laws clearly state municipalities would have this authority.
The likely policy impact is that several towns and counties will ban marijuana sales before the new law goes into effect later this year. Marijuana sales will probably be restricted to only a portion of Washington State at first. In Colorado most of the larger cities chose to adopt bans or temporary moratoriums.
Local option rules for alcohol have a very long history in the United States with 33 states allowing local governments to go “dry.” In fact there are still hundreds of dry municipalities in the country.