Ah, always around are the ill-informed chartmakers. The US is first governed by the Constitutional law, which allows the federal government to make or change law as and to the degree the Constitution permits. The US Constitution allows the federal government full authority to regulate all interstate commerce. However, it does not allow the federal government to regulate intrastate commerce, but it finds it's ways, wihin the confines of Constitutional language, to interpret such things as human rights, for instance. So in lots of situations, the Supreme Court will rule the interstate commerce clause to be applicable where the states argue it is not. Abortion is an example of a state right (I disagree) turned into a human right and therefore subject to the interstate commerce clause, allowing the US Congress to pass laws regulating it. Then the state comes in and, in reverse fashion, passes state law that eliminates funding for any abortions. But this is only an example of how the state can attempt to reverse the effect. Sorry, bad example, as most really problematic decisions end up decided by the Supreme Court.
This legal minimum drinking age thing is a perfect example of how the US Congress can, legally, manipulate the states. So I repeat, there is no federal law in America that sets a legal minimum drinking age. That is a state right.