"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?"

It is not a random "something" but something most likely accurate. True that they are not 100% sure, but if they have to choose a default value between zero and local tax rate based on zip code, which one is a better choice? Let me illustrate with an example:

When we enter a dependent. the program's default value is that the dependent lives with the taxpayer, which is a most likely case, so it is a reasonable choice. If the dependent does not live with the taxpayer, then we go in and make appropriate change.

Ultimately we are responsible for the accuracy. Would you rather see the system is 99% of times correct and we just need to make changes for 1% of the cases, or the other way around?

If the IRS can do it with its sales tax calculator, I do not know why PTO cannot do it. I often have to run the IRS calculator, and copy the numbers to the program. I simply ask to automate the process. Are you suggesting that we should lookup manually each time? There isn't much value add to the process. 

You said "Not every single ZIP code in CA has a local sales tax", that is fine. I am suggesting the PTO looks it up, and if it is zero, then we know it is zero, it is better than it does not look up and blindly put it as zero.

There should be an existing database, so it might be easier than we think, but anyway I think we should leave the implementation details to the company to worry about.