Re: New WA state insurance law??
It seems to me that it would certainly makes sense that if someone is going to to tie up for more than dinner that the marina would want to make sure that the boat had coverage. Could this be that the marina was just using it as an excuse to ask them to leave for some other reason? I don't know much about the situation, but here on the east coast you at least need to sign a form and in the form it asks if you have liability coverage. If you are staying long enough to say plug into shore power I don't think it is egregious to ask to see proof especially for a boat from out of the area. I could see it being a bit up to the harbor master to decide if they want to trust you based on your signature, but if I owned the marina I would want to at least see the policy and write down the company and policy number in case there was an issue.
Perhaps things are just a bit more laid back on the North West coast. But if I had a boat in a marina and a paying visitor (therefore the marina's customer) took out my stern rail and they did not have insurance info for him, I would be expecting the marina to paying for the damages, not to mention a propane explosion or fire.
I can't imagine this is due to a "new" law likely one that has just not been enforced in the past by more than a signature or they are using it because they just did not want to rent to them. Also the "visitor for more than 3 days" is a bit of a stretch as they were getting new drivers licenses, so they were establishing permanent residency. It likely has more to do with that than anything. Perhaps they just did not want or were not allowed more liveaboards.