1. His celebrity could increase exponentially when he returned home and was available to do a press junket and book tour. He would appear on every morning talk show and radio show and then a tour of the campuses, libraries and malls of America. This idea is absurd on its face.
2. He had the resolve to make the initial release knowing he was destroying his life - it is clear this is a moral stance for him and the personal damage is done. If he came back to the U.S., expect him to be emboldened and wind up working for a lobby or a not for profit.
3. Lobbying dollars >> optics.
RE: US Government interests:
1. It would make the democrats look weak on terror and intelligence in the face of an upcoming and obscenely hotly contested election.
2. It would be bad for morale of our intelligence agencies - by no means a 'soft' consideration.
3. It would, in fact, set a precedent that whistleblowers will get pardoned.
This last point is the whole issue. Neither this administration, nor future administrations, is going to be particularly fond of intelligence whistleblowers. Financial and business whistleblowers is one thing - but in intelligence? That is another issue entirely. All politics aside, though we may, as citizens, want this to be precisely the case and hope that if we ever held political office we would feel the same way, it is just unlikely that this attitude is going to be found within the upper echelons of the intelligence or political community.
Unfortunately, the downsides are hugely substantial to those in power and the upsides are widely distributed among the general public. With a risk/reward-matrix like that, don't expect to see any pardons coming down for Snowden anytime soon.