My understanding is that you only need to pay California state tax if you have income from California sources: https://www.ftb.ca.gov/individuals/FileRtn/index.shtml
Unfortunately you have to pay California state tax if you have income from California sources OR if you are a California resident. California has the toughest policy on expats--they often consider you a California resident many years after you leave the state.
I might be wrong, but I always thought that residency for tax purposes and voting purposes were two different things.
Here's the list of things that California looks at to determine whether or not you are still a California resident:
Amount of time you spend in California versus amount of time you spend outside California.
• Location of your spouse/RDP and children.
• Location of your principal residence.
• State that issued your driver's license.
• State where your vehicles are registered.
• State in which you maintain your professional licenses.
• State in which you are registered to vote.
• Location of the banks where you maintain accounts.
• The origination point of your financial transactions.
• Location of your medical professionals and other healthcare providers (doctors, dentists etc.), accountants, and attorneys.
Location of your social ties, such as your place of worship, professional associations, or social and country clubs of which you are a member.
• Location of your real property and investments.
• Permanence of your work assignments in California.
(That's from
www.ftb.ca.gov/forms/2010/10_1031.pdf)
I know it seems obvious that if you don't live there that you shouldn't have to pay taxes there, but California does not see if that way. If you keep your license, voter registration, bank accounts, etc. they say that shows intent to come back. A friend of mine who has been abroad for 9 years just had California place a lien on her bank account because she didn't file California state returns. I want to make sure they don't do that to me!