I think the thing Chomsky says is that it's done and it doesn't matter who did it, all speculation and energy to find the people that did it is taking energy away from other, more important issues. It's true, in a way, but then if you prescribe to that logic you will not chase any criminal for any action because it's done and it was probably someone but we have more important things to do.
We chase the criminals because you don't want them being able to do anything like this again. We need to change the structures of power so people are held accountable and so that we can minimise the risk of a re-occurrence. Notice that I haven't yet said punish, because I don't want vengeance, I want justice and closure.
Chomsky is right that (probably) every government on the planet has used 9/11 and all the other bombings and attacks to strengthen rules against their biggest adversary: the governed population of their own state.