Article III.I think they could be charged with treason. That's a capital crime.
Section 3
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.The Congress shall have power to declare the Punishment of Treason, but no Attainder of Treason shall work Corruption of Blood, or Forfeiture except during the Life of the Person attainted.
I agree - I'm guessing that the only reason they aren't being charged - yet - with treason is that it's incredibly difficult to prove in court unless you get two witnesses to testify to an overt act. And even still the appeals courts might not consider training with OBL to be an "overt" act.