I don't believe he would get a fair trial. I don't think he believes he would, either. In fact, the US refuses to promise him one: http://www.politico.com/story/2015/03/edward-snowden-wants-t...
If you think he'd even get a fair trial if he returned to the US, then that's the point you need to be justifying.
The Constitution says we get due process in trials. People in Espionage Act trials don't. Further, there's no intent by the U.S. government or courts to prosecute anyone the leaks revealed were deceiving Americans in a number of ways. Accountability works both ways. Either they all get tried for their crimes with due process or I find it highly corrupt to only want one charged while others remain above the law. I'm against him being tried unless the others are and with a defense allowed with full access to relevant, classified info. At least what's already leaked.
It's not like it's a mystery what would happen if he did. Only tyrants make martyrs.