Re: What are they thinking?
State sales taxes are regulated at the state level (surprise), and they are not uniform with each other. If you are already in a sales tax state, you are collecting sales tax on sales in your own state and 1) every state in which you have a tax nexus and 2) every state that has a cooperative collection agreement with other states.
Here in the lovely state of Oregon, we do not have a state sales tax. We do have a somewhat sizable state income tax instead. When you purchase goods through our website, we don't collect sales tax. That does give us a bit of an advantage sometimes. It also gives reason for people in border states to cross over and buy here.
Other border states are wise to this practice, and therefore have tax systems that compensate. For example, if you buy a car in Oregon, and bring it to Washington, they nick you when you register your vehicle.
As compared with many other countries, USA states have much more autonomy (and different cultural attitudes) from each other and from the federal government. Its a rough point when the EU wants US companies to cough up VAT payments. We cannot uniformly collect taxes from each other, let alone consider paying them to other countries.