Daniel Pipes is quoted on the History News Network: "This absence points to a deeper reality: the crime of treason is now as defunct as blue laws, prohibition of alcohol, or laws banning miscegenation. I predict that, short of radical changes, no Western state will again prosecute its citizens for treason."
I think he's right. We've globalized opinion, so everyone now is competing in the global marketplace of ideas. The Sopranos may still believe in death as the punishment for treason, but the rest of us don't.