It is a California requirement. To determine California nonresident or part-year resident tax, Calfiornia pretends that all of your income is taxable in California by California law. Since California does not allow a deduction for an HSA, your HSA is added back into your total income to determine this hypothetical tax. Then, the result is prorated to the actual amount of income earned in California. The HSA is subtracted for this purpose (because the HSA was not "deducted" off of California income).
To give an example: you earn 48,000 total, with 20,000 in California and a 2,000 HSA contribution that was deducted from your Federal income while working in the other state. For California purposes, your total income is 50,000, and for hypothetical purposes the tax is 1,500. Your tax is prorated to the income you earned in California (40% of 1500), so your California tax is $600.
Thus, the HSA is not taxed in California, but is factored in to determine the total amount of California tax.
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