They are literally engaging in war against their own government, physically destroying pieces of that government to protect enemies of the united states.

This qualifies for more than just a life sentence in prison. The crimes they are committing are actually the only federal crimes that are defined as punishable with execution. So yes, while punishing them appropriately for their crimes does carry a life in prison punishment, that lifetime itself shouldn't be very long. Again, purely based on the law as written, and the definition of the crimes committed, they have earned the death penalty already.