"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? : )