The issue is whether a state tax runs afoul of the Commerce Clause, U.S. Const., Art. I, 8, cl. 3, when it applies the tax it imposes on 'the privilege of . . . doing business' within the State to a party's activity in interstate commerce. In Spector Motor Service v. O'Connor, 340 U.S. 602 (1951); Freeman v. Hewit, 329 U.S. 249 (1946)., the Supreme Court looked only to the fact that the incidence of the tax is the 'privilege of doing business'; it deems irrelevant any consideration of the practical effect of the tax. The rule reflects an underlying philosophy that interstate commerce should enjoy a sort of 'free trade' immunity from state taxation. The Court summarized the 'free trade' view in Freeman v. Hewit, 329 U.S., at 252 : '[T]he Commerce Clause was not merely an authorization to Congress to enact laws for the protection and encouragement of commerce among the States, but by its own force created an area of trade free from interference by the States. In short, the Commerce Clause even without implementing legislation by Congress is a limitation upon the power of the States. . . . This limitation on State ...