So there is several things going on here. I will try to clear up a few of them.
I have worked for CDFA (not as an Ag inspector) so I know how some of these things work. I'm also a Ca farmer....or was until Ca drove me out of business with regs and.......... :(
#1. Do you have to stop at these Ag inspection stations? Answer, yes you do.
#2. Do you have to answer any questions? No you do not.
#3. Are Ag inspectors LEO? No they are not.
#4. Do Ag inspectors have arrest authority. No they do not.
#5. Can Ag inspectors call someone that does have arrest authority? Yes they can and have in the past.
#6. Can Ag inspectors inspect your vehicle without your permission? No they can not.
(the case YS quoted has nothing to do with this. In the case he quoted the LEO HAD probable cause. Just because you are traveling from one state to another does not give them PC.)
#7. Can Ag inspectors get a warrant to search your vehicle? They can try but it's super, super, super unlikely that a judge will sign off on it because you need PC FIRST!
In short, these "programs" are voluntary. If you want to participate, participate. If you don't want to participate, don't participate. By participate I mean go through searches and things like that.
In closing I find it odd that Ca politicians don't want a wall to keep out drugs and bad bug infested fruit and everything else, but want to check people coming in from other states for those things. :h :S
Here is a great video of a guy that knows his rights. This is a very funny video for me because the Ag inspector finally figures it out. :)
Anyway, enjoy.