You were already legally obligated to report online purchases that were not taxed on your state taxes. Not doing so is tax evasion and is illegal...
Yeah, and I'm going to go out on a limb and say that 99.9% of tax payers that have made online purchases have NEVER reported it. Hence why they can't really do anything about it other than making a law that requires that you pay the tax up front. I'm not a fan of the idea since I like the "tax free" shopping that I've enjoyed for years now, but it's honestly only a matter of time before a law of some sort gets passed.