Temperance related measures post-Volstead - an insidious loophole to appease the Bible Belt whereby liquour is specifically exempted from the federal oversight

Temperance related measures post-Volstead - an insidious loophole to appease the Bible Belt whereby liquour is specifically exempted from the federal oversight of interstate commerce. I believe there's some relation between this and federal highway funding being tied to a minimum drinking age of 21, but I may be misremembering some half-read magazine article.