Agriculture is a very, very important business in California. The state
is trying to protect that industry from plant pests and diseases.

With some success. For example, they've kept out the Colorado Potato
Beetle (at least from parts of the state, according to
http://entweb.clemson.edu/museum/bee...cal/btle64.htm ), despite
the fact that it is ubiquitous in most of the rest of the US.

Between the natural barrier of the Sierra Nevada (which helps make the
effort potentially non-futile), and the large number of
California-only native species (which I suppose isn't the motive as
much as the agriculture), I can see every reason for California to
worry about it.