California has explicit legislation forbidding employer interference in employee political activity: http://leginfo.legislature.ca.gov/faces/codes_displaySection....

Of course the punishment for violating this is a fine of no more than $5k, per http://leginfo.legislature.ca.gov/faces/codes_displaySection....

That said, it _may_ open the door for a civil lawsuit per http://leginfo.legislature.ca.gov/faces/codes_displaySection....

Whether any of that applies in this situation is entirely unclear to me, because it hinges on the definitions of "engaging or participating in politics" and so forth. You'd probably need to talk to a competent labor lawyer in California for anything resembling clarity here.