Can't pardon a 'non crime'
As it happens, the immunity is against civil suits for privacy infringement, which a pardon wouldn't affect. They are for criminal offenses (how about 'aiding and abetting'?).
Besides, the Govt took the amusing position that fact of whether or not the intercepts took place is in itself a state secret :-(
From my point of view enabling companies to assist the 'Guardians of Freedom' in committing crimes is a real bad idea. There is enough collusion and 'nudge, nudge, wink, wink' as it is. Worse there is general criminal immunity for acts committed in office and impeachment is all but impossible. (perhaps 'judge not, lest you be judged' ;-) )