Amazon will be taxing you in the future right? At first they wavered but then were forced to collect from us in NY for purchases on amazon.com. It's listed as "estimated taxes" but they collect that exact amount at checkout.
Why isn't it fully enforced the same for everyone / every online-only retailer? What makes us special?
Because every state has different laws regarding taxation. NY led the initial charge to tax online sales from Amazon because they were losing lots of tax revenue. NY argued that the Amazon affiliate program established sufficient physical presence within the state to justify imposing an obligation to collect and remit use taxed owed by the consumer. The legislature revised the statute to reflect it's stance and I believe NY won the battle in court, which is why Amazon collects from you.
What people are missing is that Amazon's business model thrives on avoiding of tax collection. Yes, they also have great selection and 2-day shipping, but Amazon knows damn well that consumers hate paying taxes. And Amazon couldn't care less about whether you have to pay a use tax or not. From their standpoint, being required to collect and remit use tax would add to their expenses and potentially hurt their business. Amazon knows a lot of consumers use their site instead of Walmart.com or a B&M store simply due to tax avoidance.
The vast majority of states impose a sales tax and a corresponding use tax. Some states don't (e.g. Amazon would not send you this letter had you made the exact same purchase as a NH resident). You owe SC tax based on your purchases from Amazon. Whether you wish to report them to the state is your business. But remember that state and local governments rely upon sales and use tax revenue (roughly a 1/3 of their tax revenue) and they're losing loads of money to online sales.
Currently, a state may not require a retailer with no physical presence within the state to collect and remit a use tax owed to the state by the consumer. Many people dislike this because it gives online retailers a competitive advantage over B&M and local stores and because frankly, much taxable revenue is lost because of this rule. Many states are expanding their sales and use tax statutes to capture these online purchases. Whether they will be successful is a different matter. Ultimately, the issue will need to be resolved by the Supreme Court or Congress. Congress could very easily clean up this issue, but since they have trouble getting us a balanced budget...I wouldn't count on it.