The bigger problem is that Snowden hasn't even been interrogated by the federal government, let alone tried, so it's difficult to really know what they'd be pardoning him for. Publishing the leaks is one thing, but if he's given access to foreign powers to information that has not been made public, that's another.

In the Marc Rich case, Clinton had a pretty good idea of what the upper bound of the crime being excused was. Also: he shouldn't have pardoned Rich, is I think the consensus at this point.