I agree with you in theory, but it is a matter how things should be optimized.
For example, when I enter a taxpayer, the system does not have enough information to determine whether it should use form 1040 or 1040-NR, but it does not just sit there doing nothing but assume it is form 1040 which is the most common until we instruct it otherwise.
Same here with the local tax. The most common is that the address is the taxpayer's home address and he lives there the entire year. If it is not the case, then we should intervene and do the needful. Looking up the tax rate from the zip code does not seem to be that formidable task.
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