After he knew what ? That the people he was helping had sworn to kill American's ? WTC wasn't the first time these people attacked us.
The "these people" you refer to are Al Qaeda and not Taliban, are they not? Prior to the Taliban refusing to give up OBL, where they considered the enemy?
I believe that the law recognizes the crime of being an "accessory after the fact". Perhaps Lindh is guilty of being an accessory after the fact of the WTC bombing because the Taliban refused to give up OBL and Lindh did not quit the Taliban (although he did surrender).
The challenge is to define the day on which Lindh first committed the conspiracy with which he is charged. Or if not the exact day, at least what agreement he reached on a given day. Guilt by association has never been recognized in US courts, as far as I know.