"...how does it know that the local tax rate is zero?"

How does it know that it ISN'T zero?   You'd rather have to program compute something, add it in, then you have to proactively go back and take it out?   

That leaves room for a lot of 'overstatement' of a deduction IMHO.

Not every single ZIP code in CA (where I practice) has a local sales tax added to the base, state wide sales tax.