There's a broad category of self-assessment taxes you're supposed to be paying : use tax and their ilk.
State laws almost always indicate that it is the responsibility of the purchaser to account for this. Generally, they require that you pay the taxable difference by percent between the point of purchase and the state of residence.
Example 1: You purchase a good on the internet from out of state. You pay no (sales) tax on it at the time of purchase , but if you were to buy it in your state, you'd be assessed a 10% tax. You now owe your state a 10% tax.
Example 2: You live in one state, but purchase a car in another. Autos are taxed at 10% in your state, and at 5% in the state where you purchased it. The tax for the state of purchase IS collected, but you still owe 5% tax in your state.
The reason this doesn't come up very often is that most of these sums are very very small. Certainly far smaller than the cost of litigation to investigate, find proof, and litigate remuneration of those sums. The states still want that money! They can't afford to spend tens of thousands of dollars to recover two dollars from an individual, but they're willing to spend a hundred thousand if it brings in millions. So they target specific retailers because it IS cost effective for them to force THEM to collect the sales tax as if they were in-state sellers. Thus Amazon now collects per-state tax.
The short version is that the state wants money, and they'll keep creating laws until they can both legally take it, and it's cost effective (and easy) to do so. The same idea drives universal fingerprinting/ID and encryption backdoor initiatives. There are laws that cover the situation, but they're too hard to enforce. So they add more laws simply so it's easier to enforce the other ones.