When the government came after him, they charged him with a misdemeanor. He never spent a single day in jail, despite the type of classified information he exposed.
We have a two-tiered system of justice in the United States, where people who are either well-connected to government or they have access to an incredible amount of resources get very light punishments.
I'm not so sure about Snowden's assessment of this though. Because the only thing that Petraeus did was expose information (albeit highly classified information). What Snowden did was expose a system. Yes, Petraeus may very well have merely had his
wrist slapped for what he did because of who he is. But were he to have
exposed what Snowden did there's no way he would have gotten off as
lightly, and is why there's a slim-to-none chance that Snowden is going
to see the light of (American) day anytime soon.
None of that is to say though that Barabbas should be released and that the "other guy" shouldn't be -- he should be (as should Chelsea Manning). As a recent CNN article even put it, "President Obama should suck it up and pardon both leakers before the incoming administration makes two ugly situations uglier." Touche'.
But regardless of whether or not the "other guy" gets released, there's one thing I can guarantee you, and that's that he holds no super powers that would enable him to forgive us of our sins of having built up a civilization dependent on extremely large amounts of concentrated energy and of having done virtually nothing to wean ourselves off of them after decades of warnings. For when it comes time to pay the piper, cries of privacy violations won't be standing in the Four Horsemen's way, we can be sure of that.