Excellent question. The court decision underlying all this (the "Bindley" decision) states that the independent contractor's income is "sourced" to the place where the benefit is received. If that place is California, the income is taxable by California. They will first look to the contract you have with the payor in order to determine this (if you have one).
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
