Why does TurboTax add Schedule CA (540) to a state return when the standard deduction was taken on the federal return (no itemized deductions or 1040 Schedule A)?

Many California taxpayers are able to itemize their deductions on their California return, due to the lower standard deduction even though they don't have enough deductions in order to itemize on the Federal return. TurboTax transfers all Schedule A information on the Federal return to the California return in case you can itemize for California.

Schedule CA is used to make adjustments to the federal adjusted gross income and the federal itemized deductions using California law. It lists all of the income and deduction items that are treated differently for California than on the Federal return.

See here for the instructions for California Schedule CA.

There are other things on the 540 CA than just Itemized Deductions.  I never itemize and alway get schedule CA.  There are exclusions like Social Security is not taxed on a state return so if any SS is taxable on federal it is subtracted from Calif.  Same with Savings Bond interest and some other investment interest like from mutual funds.