On the Sales Tax tab of the Real-time Setup window there is the option "Base Sales Tax on the Address From" to choose which address controls the sales tax rate. The standard is for the Ship To address to determine the tax rate locality.  

There is also the option "Show/Display USA address verification icon" to hide this feature if not wanted. The option is useful if you live in a country besides the USA like Canada. You may still ship and do business with customers in the USA, you can use this option to benefit from USA address verification even though your company is not located in the USA.