Why should online retailers be exempt? That doesn't seem very fair.
If you were talking about only State sales taxes, I'd agree, but there are also local sales taxes which can levied as well and is difficult to know which local taxes should be applied based on the billing address. Not only that, what gets subjected to the sales tax varies considerably from place to place. Sales taxes are not the simple subject some people think they are.
That's why the best solution would be for the states to agree on taxable categories and make interstate (including internet) sales only subject to the state portion of the sales tax on the agreed categories. Rates could still vary from state to state.