It's because of fruit flies. I'm old enough to remember, and it directly affected me and my family.
I'm in the Sierras now, but grew up in what a lot of you call Silicon Valley. We called it Blossom Valley back then. Because it was all orchards. Orchards we worked hard in.
The fruit flies arrived. Not a native bug for California and it decimated entire orchards. It caused several orchards around us to cut EVERY SINGLE TREE down. We lost about 75% of ours, but we lived. Well, the first year anyway.
Then the helicopters came. Spraying God knows what over entire cities and agricultural areas. I'm talking apocalyptic looking FLEETS of helicopters. It was serious. Hundreds of farms and orchards were devistated by this tiny little fly. We had curfews ordered to keep people from being outside during late night spraying. It was a bit scary really. They sprayed for days and days.
And you know what? It worked mostly. A lot of orchards and farms survived. Or began anew. For us, the cost was too much to bear and we lost it all. Every penny gone. Forced to sell the land for pennies on the dollar just to be able to break even.
So yeah, the checkpoints were VERY necessary back then, and still are today. But can you really stop every vehicle and inspect it??? No, and no one wanted to. They seriously relied on public education and cooperation back then. The education part seems to have disappeared, but those inspection stations are mostly there on the honor system of being honest and simply having a place to throw away fruit that might be harboring fruit fly eggs.
Those flies never really left by the way. They're still here. They're just manageable. The stations help them from getting out of control again.
Please, if you're traveling here, don't bring fruit in. You can't see the eggs. They won't hurt you to eat either, so you'd be surprised how much fruit you eat has those eggs in it. You probably don't want to know. But it's a real and serious issue. And was an actual emergency to the economy of California back then. Don't want to think of what could happen now