Amazon strategically places its warehouses just outside of the most populated states. Imposing sales tax would be mean they immediately lose a ~10% pricing margin on sales to states like California. That's quite a big impact.

If you look carefully at the way prices are set on some of Amazon's items, they actually set the price at just below the list price + sales tax for some populated state, such as California. So at first glance the price will seem to be higher than a competitor's, but it's actually cheaper since the competitor (i.e. a b&m retailer like Wal-Mart) has to charge sales tax.