regulating child labor was just something Congress wasn't supposed to do. Likewise, the Agricultural Adjustment Act was struck down in 1936 because regulating agriculture was something Congress wasn't supposed to do, and a certain bundle of taxes and spending were considered to be regulations of agriculture. This approach was, of course, pretty much abandoned after 1937 in all post-New Deal jurisprudence, and for good reason: it was silly, it was confusing, and it lacked any foundation in Constitutional text. How is one to know, looking at the text of the Constitution, which subject areas are or aren't appropriate for federal treatment? Moreover, if Congressional exercise of its enumerated powers is subject to such censorship on the basis of old-fashioned notions of propriety, the government will be perpetually unable to meet the needs of a changing world, not because it lacks the authority to do so under the Constitution, but because the needs are new and people are unaccustomed to thinking of Congress using its powers in that way. Categorical federalism, as it was called, was a mess, and was rightly abandoned.
But in the last third of the twentieth century, and continuing right through June 28th, 2012, the Court has developed a new standard by which to judge Congressional exercise of its taxing, spending, and regulatory powers against some notion of state sovereignty: the compulsion test. The basic idea is that Congress may not compel or coerce states to behave a certain way. South Dakota v. Dole, among other cases, upheld a law in part by finding that it wasn't coercion. New York v. United States struck down one provision of a law on the grounds that it was compulsion of state legislatures. Printz v. United States held that Congress couldn't conscript state law enforcement officers into the administration of federal regulations. And, a few weeks ago, the Court in National Federation of Independent Business v. Sebelius held that changing the conditions of a federal/state jointly-administered spending program could amount to coercion, if the dollar figures involved were big enough. In my opinion there's a tremendous amount of confusion in this whole area, mostly because the Court has declined to pursue one of the two clear, coherent paths of reasoning on the issue.