I really dont see how "hacking" terrorists will help anyone.
1) If it is just a website that you knock offline: the last time I checked, owning a website, no matter how offensive is not illegal. This is true, unless they are planning illegal activities on it, in which case, having it up is a help to anyone trying to thwart their deeds.
2) If you plan to attack a terrorists network, its servers, etc.: Good luck! If the US government can't find where the terrorist's emails are being sent, then you poking around is going to do little but arouse suspicion of your own deeds. This may well land you in guantanamo bay or wherever they are holding terrorist suspects now. If by chance the US government has found the computers where the terrorists are hiding their secret plans, then anyone attacking them and destroying the evidence will not help any cause, but rather hamper any lawful prevention of terrorism.

I really can't think of any situation where criminal activities would help thwart terrorism, online or otherwise.