What you say makes sense but they are within their rights under State Law. While RCW9.41.290 does preempt local laws on regulating firearms it does allow them to pass laws as stated in RCW9.41.300.
(3)(a) Cities, towns, and counties may enact ordinances restricting the areas in their respective jurisdictions in which firearms may be sold, but, except as provided in (b) of this subsection, a business selling firearms may not be treated more restrictively than other businesses located within the same zone. An ordinance requiring the cessation of business within a zone shall not have a shorter grandfather period for businesses selling firearms than for any other businesses within the zone.
Stupid, but not illegal. Not even questionable given the clarity of the law.