Already claim and ought to claim are two different things. Other, unfriendly, powers will return the disfavor.
Unfriendly powers already do. Iran held three hikers for over a year for espionage. They did nothing but wander across an unmarked border. Any actual American agents that are caught will certainly be killed absent aggressive intervention.
Our enemies should not dictate American policy. It is bad precedent for us to allow a foreign national to intentionally leak classified material with the intent of aiding our enemies, and to simply ignore his crimes for fear of reprisal. Reprisal is inevitable, no matter what we do to Assange.
SnakeDoc