The employer would need cause to drug test for most jobs. https://app.leg.wa.gov/wac/default.aspx?cite=357-37-200

It is definitely not meaningless. Even if some companies try and get around the rules like you are suggesting, it would be a whole lot more hassle for companies to come up with some reasoning to send one of their newly hired employees to go and get a drug test on their first day. Then, once again, come up with some bs reasoning to fire them without cause. That sounds like a recipe for a lawsuit to me.

I'm not saying it will never happen, but I definitely do not see it being as prevalent as pre-employment drug screening is currently.