You're mistaken. The California statute makes an exception for work that relates to the business of the employer. I quote from the addendum to the offer letter I received for my current job:

In compliance with prevailing provisions of relevant state statutes, this agreement does not apply to an invention for which no equipment, supplies, facility, or trade secret information of the employer was used and which was developed entirely on the employee's own time, unless (a) the invention relates (i) to the business of the employer, or (ii) to the employer's actual or demonstrably anticipated research or development, or (b) the invention results from any work performed by the employee for the employer.

That language is probably straight from the statute, though I didn't check. Anyway, it would have to be written this way, or it would be an invitation to rampant abuse by employees. As it is I think it's very fair. Note especially the word "demonstrably".