Just because they were not challenged on the law does not mean it was correct.
Another example of a preemptive pardon is Carter’s pardoning of all the Vietnam-era draft evaders.
You do raise an interesting point, tired&retired. Perhaps the Supreme Court could challenge such pardons. But I’m rather sure they won’t. A precedence has been set. And there is nothing in the Constitution to suggest that a president can’t issue such pardons.
Anyway, this is a pet peeve of mine. The Founders fought a war to get rid of a king. Then they gave the president an absolute, king-like power to issue pardons.
Something is wrong with that picture. Some sort of checks-and-balances should have been built into the process. Perhaps Congress should have the power to veto a presidential pardon just as a president can veto a congressional bill.