Upon looking at the text of the bill any reasonable person (or lawyer) could instantly find a very big reason why the 1984 NMDA Act is illegal. It's actually in 23 U.S.C. 158 which quite blatantly states that:
"(a) Withholding of Funds for Noncompliance.
(1) In general. The Secretary shall withhold 10 per centum of the amount required to be apportioned to any State under each of sections 104(b)(1), 104(b)(3), and 104(b)(4) of this title on the first day of each fiscal year after the second fiscal year beginning after September 30, 1985, in which the purchase or public possession in such State of any alcoholic beverage by a person who is less than twenty-one years of age is lawful."
You see, Section A is a problem, a BIG problem. Why you ask? Because the VERY act of "witholding funds for noncompliance" essentially bypasses one law and breaks another.
Let me explain...
The law it bypasses is the law of the Constitution, which, no where in it does it grant the federal government the right expressly or implicitly to set a national minimum drinking age. Trust me, look, there is not one law in there that allows for this type of behavior by the government. Since it does not have the authority (which it knows and therefore it bypasses the Constitution) to regulate a national minimum drinking age the only thing it can do is force the states to comply by other means.
This is where it blatantly breaks a law. In order to best understand how it blatantly breaks a law we must look at the wording of the main (but not the only) section in question. That is section A. As stated before it "withhold[s] funds for noncompliance" and when someone "withholds" something (like money) because someone (or a State) did not do something they want them to do that they would not otherwise do unless being forced to do so, there a term for this, a legal term, and what would that term be? Extortion (or alternatively: coercion, blackmail).
Oh but how can this be? According to the Encyclopedia Britannica the definition of extortion is this:
"Unlawful exaction of money or property through intimidation or undue exercise of authority. It may include threats of physical harm, criminal prosecution, or public exposure. Some forms of threat, especially those made in writing, are occasionally singled out for separate statutory treatment as blackmail."
As stated above the federal government DOES NOT have the authority to demand states set a drinking age nor does it have the power to set one itself so it places this little monetary "incentive" (albeit a negative and HIGHLY illegal one!) to get states to comply with it's "undue exercise of authority" (which, once again, it does not have the authority to set the age or even make the states set an age so this more than falls under the "undue exercise of authority").
So my question is how the hell did this even get passed in the first place? One answer: Lobbying by some VERY angry bitches. True story. Look up MADD. I've mentioned them before. They are quite angry bitches. And you can quote me on that. ;)