The various states have different methods of allocating resident and nonresident income for the purpose of calculating taxes for nonresidents. Depending on an individual's situation, one state's method may appear more inequitable than another.
California's method starts with figuring California tax on all your income, figuring the percentage of your income that was California-sourced, and applying that percentage to the tax calculated on all of your income. So, you aren't being taxed on all of your income, just the percentage of your income that coincides with the percentage of your income that was California-based.
Here's an excerpt from California FTB Publication 1100:
For taxable years beginning on or after January 1, 2002, if you are a nonresident or a part-year resident, you determine your California tax by multiplying your California taxable income by an effective tax rate. The effective tax rate is the California tax on all income as if you were a California resident for the current taxable year and for all prior taxable years for any carryover items, deferred income, suspended losses, or suspended deductions, divided by that income. Use the following formula:
Prorated tax = CA taxable income × Tax on total taxable income ÷ Total taxable income
The publication does state "as if" you were a California resident. I hope this is helpful in clarifying the basis for the calculation.
@Sedes Gobhani
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