Given that several senior US politicians, including at least one who ran for the presidency, have openly advocated the use of "enemy combatant" status at home[1], it's not difficult to see why someone like Snowden might not entirely trust any promise of amnesty.
As someone from outside the US, I found that particular advocacy (and the rationales claimed to justify it) one of the low points of recent years. It makes sense that you might need different rules for acceptable treatment of enemy combatants when you're in combat, when you don't have the luxury of time and calm deliberation that a court affords because people are shooting at each other. But to me, the appropriate response to recent "terrorist" attacks is to treat them exactly like any other threat, assault or murder, and to put suspects/perpetrators through exactly the same legal processes as anyone else. It is disturbing that we are even considering the possibility that someone who isn't accused of any violent crime at all might have to worry about being labelled an enemy combatant and thus forfeit their normal legal right to due process.
[1] http://www.nydailynews.com/blogs/dc/2013/04/king-mccain-grah...