I was always under the impression that the rule used to be here in the States (although who knows what rules are now)that if an Online retailer had a physical presence in a State that they had to add the Tax i.e. Amazon building Fulfillment Centers in your State. Or there is a brick & mortar store or warehouse. If no physical presence then it would be up to the buyer to declare purchases on there yearly taxes. Since Steam has no physical presence anywhere and I think based in another country (could be wrong) that is why I am so surprised.
@Oldbroad
I agree, paying what one owes in taxes is always the right thing to do. But my opinion, the easiest way here in the States, would be for the seller to charge the buyer the Tax rate from the State that they are in which across the board would not place a burden on Retailers having to keep track of all the different States tax rates, who purchases from where and then sending different payments to each State. In my way of thinking, if I want to purchase items from a State that has a lesser tax then that would put the States in more competition with each other to bring their tax rates down for Online sales and/or encourage more Online Retailers to move to their States so they may add to their coffers. It just seems so simple to me.