The TurboTax guidance says that to make sure that you don't claim a combined total of deduction and PTC that is greater than premiums paid. Marking the box causes TurboTax to do its iterative calculation which ensures that. This prevents undesirable feedback from the IRS (and claims of TurboTax error that could cost Intuit money) but might not give the taxpayer the best taxable result.
Even if TurboTax implemented a check to make sure that the sum of the deduction and the PTC did not exceed premiums, TurboTax would not be able to tell you how to adjust the deduction to correct that and would just leave the user hanging with a tax return that could not be filed.
