Ugh. We have had similar problems here with other respiratory diseases, and similar heartaches.
Newcastle is problematic because the virus can survive in frozen material almost indefinitely, so it is not unheard of to ban imports of chicken from an entire region or country to preserve poultry health in the importing country (i.e. CA stands to lose buckets of money if Asia stops importing eggs or chickens from the state, for example). They've been trying to stamp out Newcastle in CA for a few years now, because similarly, if you have an area free of a certain disease, imports go up.
There is no treatment, just euthanasia, and the only prevention is vaccines. I imagine CA is like here, some people here are resistant to vaccines, it's a complicated situation. I imagine the law focuses on eliminating the greater risk to the population and state as a whole (which seems similar in cases like recent measles outbreak, for example, large-scale risk versus personal choice). But if I had a backyard flock I bet I would share your conviction.
Edited to add: it probably would help a lot more, big picture, to undertake a PR campaign about how to eliminate the virus (solar irradiation, dehydration) and clamp down on introduction of smuggled wild birds and other vectors. How the birds are killed doesn't matter nearly so much as what is done with their environment to stop the cycle. Maybe there is a lot of public outreach going on, for all I know.