[throwawaygulf]

Even this is not entirely correct. They do not consider you a CA resident, but they consider the income you make while (more than transiently) in the state taxable.

Many states have similar clauses (NY, NJ, etc.), CA is not unique in this aspect.

[dragonwriter]

CA is actually more generous than a number of states, which do not exempt transitory presence for work, so if you do earn any money at all for work while in the state you owe income taxes (and your employer is obligated to withhold for the State.)