Three more states have joined the growing list where you'll be charged sales tax on Amazon purchases: Indiana, Nevada, and Tennessee. Amazon already collected tax in 16 states, and in 2016, South Carolina will join them, bringing the number up to an even 20. Technically speaking, you're supposed to add up purchases on your tax returns (the "use tax") no matter where you are, but that oft-ignored rule has increasingly given way to automatic point-of-sale charges. This hasn't happened without strong pushback from Amazon and other online retailers, though; they've gone through several long legal slogs as states pursue sales revenue and parity for local brick-and-mortar businesses.
Amazon's warehouse expansions have given it a physical presence in more and more regions, speeding up deliveries but also opening it up to taxes, but pulling out of a state isn't necessarily a panacea. Recently, the Supreme Court declined to hear an Amazon lawsuit against New York, after the company attempted to fight a ruling that its relationships with local affiliates constituted a physical presence. Though it opposes what it calls a patchwork of state-level taxes, Amazon supports Congressional efforts to establish nationwide online sales tax rules.