That's DoJ policy. The legal community in the majority also agree with that, but, here is the text from wikipedia:
> The Constitution grants the president the power to pardon "offenses against the United States".[5] An offense that violates state law, but not federal law, is an offense against that state rather than an offense against the United States; however, the Supreme Court has never ruled on this matter or in the President's power to grant a habeas corpus petition for a state offense where it has been denied by a federal court.
If you consider english common law and the absolute nature of the pardon power, including presumptive crimes then this is an untested grey area. If the power was limited, the limit would have been stated. The current consensus is that the limit is implied, but an argument can be made that since english kings had pardon power over their whole domain, the same is implied when stating "offenses against the united states".
Federal authority supersedes state authority in every other case, if this was different it should have been stated so as it is a deviance. But ultimately this would come down to a supreme court opinion.
> The Constitution grants the president the power to pardon "offenses against the United States".[5] An offense that violates state law, but not federal law, is an offense against that state rather than an offense against the United States; however, the Supreme Court has never ruled on this matter or in the President's power to grant a habeas corpus petition for a state offense where it has been denied by a federal court.
If you consider english common law and the absolute nature of the pardon power, including presumptive crimes then this is an untested grey area. If the power was limited, the limit would have been stated. The current consensus is that the limit is implied, but an argument can be made that since english kings had pardon power over their whole domain, the same is implied when stating "offenses against the united states".
Federal authority supersedes state authority in every other case, if this was different it should have been stated so as it is a deviance. But ultimately this would come down to a supreme court opinion.