That which has as its subject the lone individual acting in isolation of all others cannot be the subject of a just law.
The need for equal application and protection of law, can only consider two or more persons by the very nature of meaning of the term "equal". Any other construction becomes meaningless.
Under the Constitution, that which impacts persons in plural as in "common defense", or "general welfare" is the subject of national government. All else is beyond the reach of national taxation and must of necessity go through the state or left to the individual by the very wording of the enumerated powers of the Constitution in Article I Section 8 and the 9th & 10th amendments to the Constitution upholding that interpretation.