Pensacola gunman got around a ban on foreigners buying guns

I cannot come to grips with the notion that someone who is a citizen of another country deserves, only because of that, to be put in a category of holders of lesser rights than the rest of us. For a country supposedly committed to equality for all, why is there a seeming a footnote that reads in part "unless you are non-citizen, in which case you are discriminated against in ways that would never survive 14th Amendment challenge if the case was about US citizens?