They should have been tried long ago.
One big difference is they didn't join AlQaida and they weren't running about in Yemen with AlQaida armed military personnel.
At the moment they are nearing retirement and aren't enough of a threat for anyone to worry about. You could send either one of them a subpoena in the mail and they'd respond. That wouldn't been likely with the AlQaida puke.
Both Kerry and Fonda committed treason and ought to have been tried. As citizens they deserved that.
Had they remained in Hanoi, it would have been another story; had they acted as members of the Hanoi leadership (or had they taken up arms) they would have been legitimate battlefield targets.
Like Awlaki and Gadahn.
If Gadahn wants the protections his citizenship affords he needs to get away from other legitimate targets of war, and he needs to turn himself in for trial (if Justice wants to charge and try him). If he remains in the theater of war and remains in the company of declared enemies, and continues to act as one of them, he's a target.