[btilly]

I can't find a justification why online retailers should be exempt from sales tax;

This justification is to be found in the US Constitution where authority over interstate commerce is granted to Congress. This leaves states with no legal authority over interstate commerce.

This reasoning is preposterous; the sales happens where the customer is, and that's where the tax is due. That's what will happen eventually; they are just buying time -- and not very elegantly.

In theory, yes. In practice, no.

Experience says that people don't voluntarily report or pay taxes on those sales to the state government. Furthermore state governments have proven unable to track down those sales without the cooperation of merchants. So if Amazon doesn't cooperate, those taxes don't get collected.