You do know that in the US counties (and possibly cities, I need to double check cities) can leverage sales tax as well. However, it should only really be an issue of where you have nexus.
It's probably just how their system handle's orders (you need home address for CC for AVS, so it's probably just required everywhere in their code.
In the US, cities can levy sales taxes as well. Also, the city field in your address does not accurately indicate your municipal boundaries; it's more of an identifier of which post office serves you and which municipal boundaries it falls within.
