These are the 3 examples that I tend to put into letters and emails that I wright. I emphasize the money part of it the most, pointing out that a wrongful death could be in the millions of dollars.
1...With the law on the books, you could still be asked to leave or arrested/cited under the code. Unlawful? You bet! But with the law on the books they can still arrest/cite you and "let the prosecutor sort it out" as a lot of cops like to do. This can lead to a civil suit for someones rights being violated, with tax payer money being shelled out in the end.
2... Its a liability for the municipality (county/city). If someone who is not as well versed in the RCW's as people here are, there is the potential for them to see said signage, leave their firearm in the car and have something happen to them while they are there. They can then turn to the municipality and sue them because their code is unlawful, and had they been able to have their firearm there might have been a totally different outcome. Imagine the amount of money out of the tax payer coffers for a wrongful death suit over a preempted code that they not only knew about, but just didn't feel like changing it. All it would take is for the emails and letters they send in response to someone pointing out their code is unlawful, and it would show they acknowledge that they are in fact wrong but just don't want to bother changing it.
3...The public opinion and example being set by the guy with the gun ignoring the sign... makes people that OC look like we just do what we want, and when we want and laws be damned. The problem is that they don't know that it is not enforceable, unlawful and preempted. So there ends up being one more person out there that is anti-oc because their perception is that we "carry guns, and do what we want."
The list goes on from here... I am sure that we can come up with many reasons that a unlawful code is a problem... But I have found that using these three examples in letters gets their attention, 1 and 2 the most because of the amount of money that could potentially be paid out to someone.