Here is a quote from Burdick "A pardon is an act of grace, proceeding from the power entrusted with the execution of the laws, which exempts the individual on whom it is bestowed from the punishment the law inflicts for a crime he has committed." It's clear that if you accept a pardon you are admitting you have committed a crime.

Gerald Ford used Burdick to justify his pardon of Nixon, using that portion of Burdick that puts forth "that a pardon carries an imputation of guilt and that acceptance carries a confession of guilt".