By accepting the pardon the accused concedes to guilt in the crime.
This is not necessarily true. In Burdick v. United States it does say "an imputation of guilt and acceptance of a confession of it" but there is debate about whether it is binding of not.
Apparently, there is something in Lorance v. Commandant, U.S. Disciplinary Barracks that indicates that accepting a pardon does not imply guilt, but I am not very knowledge on that.