However, all US sales taxes are state or city taxes, and there is no federal sales tax, which is a big part of why sellers are not required to collect it in online sales - its nearly impossible for them to determine what jurisdiction you're making the purchase from, since it's not only state but municipality that matters.
In the US, the federal government is funded primarily by income and capital gains and corporate taxation, states with a mix of sales and income taxation, and municipalities and local jurisdictions (like school districts) with real property and sales taxation.
edit: also, state-mandated sales taxes in the US never exceed 10%, and I believe 6% is the most common number. Municipalities often implement higher, but item-specific sales taxes. For example, Washington DC has a 15% sales tax on restaraunt purchases. Generally, these taxes are designed to strip money from tourists and the things they buy (hotels, restaurants) and avoid punishing actual residents, since the residents elect the politicians who implement them.