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.
Yes it is. It appears that there are witnesses. If the training is intended for use against American citizens it should be considered an overt act. I think there would be a stronger case to be made against this group than John Walker.