Zakarias Moussaoui was no where near New York City on 9/11. He was not in a plane. He did not hijack a plane and fly it into a building. He didn't help the people that did fly hijacked planes into buildings that day. He couldn't have done these things, he was in jail.
Moussaoui is either stupid or crazy or both. Listen to him for a few minutes and it's obvious. Nobody would trust him to hold up his end of a secret plan to attack America. Or even know the plan. He didn't spill the beans before the attack because he didn't have any. He hasn't enlightened US much since the attack. He doesn't know.
He may have actually been recruited by Al Qaida, he may have thought that he had a mission, he may have been deluded or misled with some messianic vision of a divine call to action. But he hadn't been told what it was.
If anything, he was a liability to the plotters, maybe a loose cannon, certainly a loose end. And in such a high stakes exploit, a disgruntled volunteer, turned away at the gate, could bring it all down if he cries on the wrong shoulder. So let him think he's on his training and get him out of the way. Flight school in the US was too exposed, he got grabbed. He's in jail, but he can't hurt them much. Stupid git.
He associated himself with a group that attacked US. He was not part of that plot. How is he guilty of conspiracy? Even if he had intent, he didn't have a plan and that is a pre-requisite for PRE-Meditation.
Now he's had a long time to cast himself, at least in his version of reality, as a Martyr. He's been held in the Great Satan's Dungeon, awaiting execution for four years now. He has nothing to lose and everything to gain by identifying himself with the 9/11 plot. He can romantically die challenging Satan in his lair! He'll go down in history with Saladin!
Only he was sentenced to life in prison. No hero's death. No glory.
This was a wise sentence, considering the smell of blood in the air. It demonstrates the strength of our judicial system, that someone associated with such a heinous crime, but not responsible for it, would be punished as a criminal, not elevated to Martyrdom.