There are/used to be checkpoints at the CA/OR border as well....

So what if a state wants to "protect" itself? The state of Western Australia, for example, is almost militant about what comes in from other states, with checkpoints at land borders, and "agricultural customs" for domestic flights from other Australian states. Flying in from Victoria I had an apple that would have been a no-no. Had to eat it on the plane.... Thus far WA has kept out many of the "dangerous pests" you'd find in other Aussie states.