We sell digital services (ebooks count), and require the customers address (or part of) to determine the VAT status of the sale.
Even though buying an e-book for Bitcoin seems like you shouldn't need any personally identifiable information, quite often merchants do need to collect it.
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.
Even though buying an e-book for Bitcoin seems like you shouldn't need any personally identifiable information, quite often merchants do need to collect it.