Inspection Stations on Interstate Highways??

I have driven in all 50 states and crossed a state line to get to all of them with the obvious exception of Hawaii (and Alaska where I got the usual US/Canada customs formalities), and California is the only state I found that has agricultural inspection stations. I am always reminded of the Joad family in The Grapes of Wrath...

For the record, the checkpoints aren't just in the desert. Less than a month ago I had to stop while coming down US 101 from Oregon north of Crescent City. The guy just waved me through, however. I drive a small car with Washington plates, so he probably figured I was harmless.