The basis for pre-emptive pardons being unlawful can be found in ancient English constitutional law, see the Prince of Orange’s 1688 Declaration of Reason that preceded England’s 1688 Bill of Rights. This carries over to USA via the 9th Amendment “The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.” A pre-emptive pardon also violates due process of law, see 5th & 14th Amendment and can therefore only be done after conviction, see this passage from the Prince of Orange’s (Future King William IIIs) Declaration of Reason.
“For though the King may pardon the Punishment that a Transgressor has incurred, and to which he is condemned; as in Cases of Treason or Felony; yet it cannot be, with any Colour of Reason, inferred from thence, that the King can intirely suspend the Execution of those Laws relating to Treason or Felony, unless it is pretended, that he is cloathed with a despotick and arbitrary Power, and that the Lives, Liberties, Honours, and Estates of the Subjects, depend wholly on his goodwill and Pleasure, and are intirely subject to him; which must infallibly follow on the King’s having a Power to suspend the Execution of Laws, and to dispense with them.”
Hellbound and Down 