He doesn’t technically have the unilateral power to prevent future people from being charged federally with simple possession, but in reality, he kind of does (while he’s president).
Sends a pretty clear message to any federal prosecutor that is deciding if they want to charge this. Obviously, the prosecutor can spend the time and effort on this if they want, but it’s time that will probably be better served doing something else if they don’t want to see their work wiped away by the stroke of a pen in the future.