Wouldn't you expect to see some states to not comply then?

Interesting case law the above poster is referencing I think: https://en.wikipedia.org/wiki/South_Dakota_v._Dole

Notably one of the dissenting judges said this: "establishment of a minimum drinking age of 21 is not sufficiently related to interstate highway construction to justify so conditioning funds appropriated for that purpose."

It did however pass the supreme court at 7-2.