Whether a tax is to be apportioned among the States according to the census taken pursuant to article I, section 2, or imposed uniformly throughout the United States depends upon its classification as direct or indirect.[251] The rule of uniformity for indirect taxes is easy to obey. It exacts only that the subject matter of a levy be taxed at the same rate wherever found in the United States; or, as it is sometimes phrased, the uniformity required is "geographical," not "intrinsic."[252] The clause accordingly places no obstacle in the way of legislative classification for the purpose of taxation, nor in the way of what is called progressive taxation.[253] A taxing statute does not fail of the prescribed uniformity because its operation and incidence may be affected by differences in State laws.[254] A federal estate tax law which permitted a deduction for a like tax paid to a State was not rendered invalid by the fact that one State levied no such tax.[255] The term "United States" in this clause refers only to the States of the Union, the District of Columbia, and incorporated territories. Congress is not bound by the rule of uniformity in framing tax measures for unincorporated territories.[256] Indeed, in Binns _v._ United States,[257] the Court sustained license taxes imposed by Congress but applicable only in Alaska, where the proceeds, although paid into the general fund of the Treasury, did not in fact equal the total cost of maintaining the territorial government.
The discretion of Congress in selecting the objectives of taxation has also been held at times to be subject to limitations implied from the nature of the Federal System. Apart from matters which Congress is authorized to regulate, the national taxing power, it has been said, "reaches only existing subjects."[258] Congress may tax any activity actually carried on, regardless of whether it is permitted or prohibited by the laws of the United States[259] or by those of a State.[260] But so-called federal "licenses," so far as they relate to trade within State limits, merely express "the purpose of the government not to interfere * * * with the trade nominally licensed, if the required taxes are paid." Whether the "licensed" trade shall be permitted at all is a question for decision by the State.[261] This, nevertheless, does not signify that Congress may not often regulate to some extent a business within a State in order the more effectively to tax it. Under the necessary and proper clause, Congress may do this very thing. Not only has the Court sustained regulations concerning the packaging of taxed articles such as tobacco[262] and oleomargarine,[263] ostensibly designed to prevent fraud in the collection of the tax; it has also upheld measures taxing drugs[264] and firearms[265] which prescribed rigorous restrictions under which such articles could be sold or transferred, and imposed heavy penalties upon persons dealing with them in any other way. These regulations were sustained as conducive to the efficient collection of the tax though they clearly transcended in some respects this ground of justification.