You make a good point, but this is really a conundrum. To be particularly useful in California, a book has to be regionally specific to California. The vegetative communities of California vary dramatically from that of all other parts of the country, and California has far more endemic species than any other state. That is why California tends to have tree, shrub, wildflower, and other field guides specific to it. The only way I could have catered to California edibles is by including lots of plants that are unavailable to the vast majority of Americans. It is an ecological and geographic reality that most people in smaller bioregions understand. The largest bioregion of the US is the eastern forest, followed by the great plains, followed by the rocky mountains, and these three bioregions encompass the vast majority of the land area and population of our nation. Naturally, a book focusing on the whole country will include primarily plants of these bioregions and tend to ignore plants that have a limited distribution in only one smaller bioregion. Especially if the book only cover say, 32 out of 500 edible plants.
That being said, just under half of the plants in the book are found in California, which is fairly representative of California's uniqueness. I understand that the book is more useful in other parts of the country, but that doesn't make it useless where you are. Christopher Nyerges' "Guide to Wild Foods" has a clear bias for California plants that is not at all revealed in the title, yet that doesn't bother me a bit. I am glad he kept to the plants that his own experience made him particularly knowledgeable about.
That being said, I understand your complaint--it has certainly been made by others. I just don't know of any good resolution to it, other than a very cumbersome subtitle. The common solution to this problem, of course, has been for authors to include plants about which they have no or virtually no firsthand knowledge, and I didn't want to do that.