This state feeds lots and lots of people. Even people who don't live in California. We may even grow rice for Japan. As a result all those growing things inside the state, the Department of Agriculture is very paranoid. They don't want plant diseases to decimate the plants in all those fields.
People get sick.
If it's night, there are biiiig, bright lights shining over that little section of the freeway. They're there to spot check and catch people who are smuggling illegal aliens who are coming from Mexico and points further south. How they're able to spot aliens, I don't know. I've heard that California is concerned about diseases or insects that destroy the harvest. We were asked the same questions at the airport.