Sentencing for any crime is up to the judge, who is given a range of possibilities and may or may not be influenced by factors you raise in justification such as level of damage - it is not quite correct to imply that a judge must "justify" a life sentence in this or any other case. If it is within the window he or she may invoke it for any reason he or she desires, or none at all.
Pollard is something of a special case in juridicial matters, as is any employee of the federal government entrusted with access to sensitive information. These - I was one - cannot be treated quite like a regular citizen in a number of regards, not the least of which is testimony to pertinent matters in open court. There are restrictions on First, Fourth, and Fifth amendment rights that do not occur with persons accused of a normal felony or misdemeanor.
But the bottom line is that what he was doing was knowingly breaching a trust with known penalties - Pollard himself has said as much, for heaven's sake! Anyone who is in this little game has friends who have vital interests in the information they are safeguarding but who are not cleared for access. It goes with the job, and it is one of the first and most important things you're briefed on when accepting this level of responsibility. If you fail the trust you may expect to have the book thrown at you, period. Anything less than that isn't a matter of "fairness," it's a matter of luck. Pollard understands that as well.
I don't blame his friends, supporters, or the people that benefited from the release of this information, from speaking up for him. Good folks, all, but they don't understand the game and they don't understand the rules and Pollard does.
Do the crime, do the time isn't a difficult concept to understand. However when my Justice Dept enters into a plea agreement (and I don't much like them), I do expect them to keep their word. And when they don't, based on secret information, it concerns me. IMO, that's the issue here, not Pollard the individual. And it's an issue which could be easily resolved by declassification of the 4 page communication to the judge.