The truth is, the Government is not sure "how" to apply taxes to purchases made on the Internet. There are many reasons that apply to this, with a major reason being:
A customer can be placing an order on a plane flying overseas, while his billing information is in California, and the order is being shipped to Canada. How would one tax this transaction? Believe it or not, this transaction can be taxed by every state that the data "travels" through. Luckily, this is not the case.
It is also "mandatory" that online shoppers report every purchase they make on the internet. However, this is very rarely enforced, as one's total amount of purchases made online are extremely tough to track efficiently.