It's like the law where if you ride your bike on the road or sidewalk. It's 100% up to how busy the cop is that day and if he REALLY wants to write you a ticket.
Also, because this is the US, and Seattle in particular, police discretion means that some populations get the brunt of enforcement.
Nearly half of helmet citations in Seattle are against homeless people. And that was where they could corroborate homelessness using easily identifiable records (noted in press, police reports, or the address on file was a homeless shelter) https://crosscut.com/news/2020/12/nearly-half-seattles-helme...
