Agreed, its not if, but when. Both federal and state lawmakers are aching to find new revenue streams and for states taxing internet sales is a tempting solution. Especially when the consumer is technically on the hook to pay the the tax when its not collected
This does enter a legal morass which will take a while to play out.
Legally, Amazon could create 2 entities: one which sells products exclusively to 49 states (let's call it Amazon 1), and another that sells products to the state which houses Amazon 1 (let's call this one Amazon 2).
Under current law, it would be impossible for any state to charge sales taxes because the Amazon entity selling to that state wouldn't have a physical presence there.
Use taxes are notoriously difficult to enforce and are essentially worthless for low value items because the bureaucracy would cost more than any potential revenue.
The only way this can be worked out is if we get a federal statute that gives each state the right to collect those revenues. In the current anti-tax climate, that's probably very unlikely.