Conservative groups influential with the Trump administration have pressed for the preemptive acts of clemency.
There’s a world of difference between a pardon issues to prevent malicious prosecution for non-crimew and one issued to people you’ve been instructing to commit crimes



After charges. You can pardon from a conviction or a charge but you can’t grant preemptive pardons because those are effectively immunities.
You are in error. Nixon was pardoned by Ford for any and all crimes he may have committed in office. Carter issued a blanket pardon for all Vietnam draft dodgers whether charged or not. There is nothing in the constitution or court rulings that blocks a preemptive pardon.
The only rule, from an 1866 SCOTUS ruling is that the pardon must come after the crime, whether there have been charges or not.
Edit to add: Trump’s pardon of the Jan-6ers included a preemptive pardon for all those that had not been charged yet.
A pardon was issued, there’s still an argument about how legal it may or may not be and the consensus is that it would not survive legal test.
You can’t pardon someone without them admitting a crime. There is no blanket pardon. The fact no one has tested it does not make it legal but simply untested.
It’s only ever been tried ex parte and they said only federal charges. Any state can try treason.