"Can someone please explain to me why we don't put people on trial for treason anymore???"
Here is a pretty good explanation from Daniel Pipes.
http://www.danielpipes.org/article/2865
Why this collapse? Because the notion of loyalty has fundamentally changed. Traditionally, a person was assumed faithful to his natal community. A Spaniard or Swede was loyal to his monarch, a Frenchman to his republic, an American to his constitution.
That assumption is now obsolete, replaced by a loyalty to one's political community – socialism, liberalism, conservatism, or Islamism, to name some options. Geographical and social ties matter much less than of old.
So if that's the case, and we can't charge treason, but we're in power...can we deport them? : )