How soon before state governments start claiming that these lockers are a physical presence of Amazon's retail operations?
My guess is that the answer to "how soon" would be 'the second one of these boxes appears in a state where Amazon does not yet collect sales tax.'
That said, I'm guessing that Amazon's lawyers would argue that this is effectively identical to a P.O. box, and therefore totally exempted. At least, that's what I'd argue.
edit: or not owned by Amazon at all. Good call.
Make it a joint venture with UPS and use it for all kinds of deliveries and win.
In any case, there's no way Amazon would keep these around if they alone necessitated a sales tax, since the trade-off would be too great.