+1

Of course I know about probable cause and such, but I highly doubt an "agriculture" stop would necessitate automatic probable cause for each car stopped. And the refusal to give the officer permission to search is not probable cause.

Probable cause is what it makes it possible to turn a stop into a search. If there are exigent circumstances, a warrant isn't required.

You have very few Fourth Amendment rights when it involves anything on wheels. Usually, there is some sort of "reasonable suspicion" required to justify a stop, but DUI and immigration checkpoints are exempt from that standard across the US, while California has also exempted its agricultural checkpoints.

The Cal--Ore border check point is about agriculture only. California's main industry is still agriculture and contaminated fruit or veggies could harm local crops which wouldn't do the farming industry any good. There is no restriction on the movement of booze anywhere just so long as you don't have an open bottle of the stuff in the car

Isn't Oregon's main crop Marijuana now?

I've been through the Cal-Ore check point so many times over the years. Usually they just ask if you're carrying any fruit or veg and if you answer no you're on your way. Some vehicle passing through might raise a flag for whatever reason and they could be searched.
Your fruit and veg will be confiscated and that's all there is to it. Refusing a search is pointless unless you like the idea of the Highway Patrol coming along to ask questions

There are some small communities up north along the Cal coast where growing Mary Jean is a major part of the local economy

Everything RWFLP said, plus law enforcement officers can ask whatever the hëll they want (First Amendment and wot not). They may, or may not, be able to tell/force you to do something.

They're looking for crooks. I'm not one, so I have no interest in starting an argument. I suspect that most people think the same way.

Whenever I have been stopped at a checkpoint I have always been cooperative, provided my license, and exchanged a few pleasantries. Once an officer commented on my accent, but only in a neutral/ positive way; I just don't think he expected the driver of an F250 in a small town in NC to have a Yorkshire accent.

Why's that funny?

Dunno, unless he thinks that it indicates that I do it by hand? All our vehicles have electric window winders.

Not sure, but I think Americans might roll down the window, although there isn't, and never was, any "rolling" involved in opening a vehicle's window - the glass is a bit too stiff to roll!

We've got two cars with manual window winders. Nowt funny about them when the sky opens in biblical form and you're looking for the fricking button to prevent drowning.

Me too, our car has manual window cranks and no central locking. If I want to open or lock a door from outside the car, I get out the key & turn it in the lock. Don't really see the point of automated bells and whistles on cars. (2009 Yaris.)

Because I laughed. Do I have to explain why? Sorry if I offended anyone. Most people say open and close in regards to car windows these days, out in the sticks maybe different ...