dude, while in principle i agree with you, enlisting in an enemy army and shooting at US troops makes a trial redundant. There is no misunderstanding finding you in an enemy uniform. A trial is a very nice and useful thing, used to determine if you maybe are not an enemy. There is no possibility at all of this happening if you are caught actively fighting in a war. And let's be clear, this wasnt some office clerk abusing power - like that horrible woman who refused to wed a gay couple. There is no chance of finding that accepting this then turns into something that could affect you; it needs to be very much deliberate to act in such a way that your citizenship is revoked.
Although, i gotta say, if she was to come back it would only be to go to jail forever, but i'd rather that she died in the desert.
By the way, there *is* such a thing as renouncing citizenship. And while she may not have formally requested it, her actions are more than enough for the government to "assume" she was ok with that.