Yeah, it was a little questionable how they evaded collecting state sales tax in the U.S
More to do with reinterpretation of the laws on taxing mail order business. The states suddenly started arguing that if one person in that state had an Amazon affiliate link on their website then all sales to customers in that state were internal sales and not out of state mail order.

I doubt if Amazon are totally unhappy as they can deal with the additional book keeping more easily than smaller competitors can.