I think we have established The Answer.
An ag inspection is voluntary(per law cited above) and the ag inspector has the legal authority to deny you entry to the state at his whim.
If that happens, you can:
1) turn around and spend your tourista dollars elsewhere; or
2) go find another, less traveled road into CA.
There are other options, of course, like showing your rear and being mouthy. Cant say I recommend those- the more you talk, the worse it potentially gets both from a self incrimination and pi ssing off the officer points of view.
My advice is to Politely but Firmly refuse a search if you so choose. Don't argue, don't debate, don't whine.