The Fourteenth Amendment was intended to provide broad protection from legislation that denied rights and privileges to segments of society. The particular case that led to its passage was legislation in various southern states denying African Americans the right to vote. It’s unfortunate that the drafters limited the scope of the Amendment to States. As things stand now, any strict constructionist would certainly throw out any attempt to extend the applicability of the Fourteenth Amendment to federal legislation, in effect creating a giant exemption in the equal protection clause for the federal government to slip through unimpeded.
No doubt, this limitation was not lost on anyone at the time - in politics it’s often necessary to draft compromises in order to generate some favorable effect, even if it doesn’t have universal applicability. This is exceedingly unfortunate - had the drafters been a little more visionary or perhaps had the political capital to pass the Amendment without the giant federal loophole, then one could certainly argue that tax treatments that put one class of society at an economic disadvantage to another, violate the equal protection clause in exactly the same way that land ownership as a prerequisite to voting rights denied emancipated slaves the right to vote.
Wouldn’t that be something … the Supreme Court overturning regulations regarding differential tax treatment of earned income and capital as a violation of Equal Protection. Perhaps that’s exactly what’s needed, an Amendment to the Fourteenth Amendment to remove the limitation that it only applies to States. Assuming that such an innocuous Amendment passed, revisiting the Sixteenth Amendment in the light of a broadened Equal Protection clause should have legal traction that even strict constructionists couldn’t ignore.
Now that’s a plan of action…