This is not true. The courts found that a president can pardon a supposed crime before charges have even been filed, as well as any time before or after trial/conviction.
This was the case with Ford's pardoning of Nixon, as well as Carter's pardoning of all those who avoided the draft during the Vietnam war.
"There are substantial differences between legislative immunity and a pardon; the latter carries an imputation of guilt and acceptance of a confession of it,[..]"
You can only be pardoned if you have committed a crime. You have to either confess or admit your crime, or be tried, or at least be charged with a crime.
Nixon confessed and admitted that he was guilty. We wasn't tried, maybe not even charged, but he admitted wrongdoing.
As far as I know, Snowden has not admitted that he's guilty of a crime, he hasn't been tried, and he hasn't been charged. So from a legal point of view, he hasn't committed any crime and cannot be pardoned.
Good point! I was under the impression that he had admitted that he was guilty in some form. It's unclear to me now if he did. I don't know if anything he's said and done in the aftermath may be considered an admission of guilt. So my previous comment might be incorrect.
It's also unclear if a pending impeachment might be considered a "conviction" if it's just a likely outcome and not an actual impeachment.
This was the case with Ford's pardoning of Nixon, as well as Carter's pardoning of all those who avoided the draft during the Vietnam war.