They say they did so because the president has those "special war powers" under Article II of the US Constitution.
The story goes that in time of war, since he is commander in chief, he does not need congressional approval or judicial approval to do wiretaps or other searches.
That is not the general legal consensus however.
Be that as it may, remember that before 9/11 there was no war and therefore no special super duper presidential war powers. There was no war till quite a while after 9/11.
We have discussed spying on Americans and other intelligence gathering here, here, and here.
Now there is sworn testimony in court that such spying on Americans was happening well before 9/11:
Startling statements from former Qwest CEO Joseph Nacchio's defense documents alleging the National Security Agency began building a massive call records database seven months before 9/11 aren't the only accusations that the controversial program predated the attacks of 9/11.(Wired). The complaint filed in one court case, at paragraphs 81 and 82 alleges:
81. Within eleven (11) days of the onset of the Bush administration, and at least seven (7) months prior to the attacks of September 11, 2001, defendant ATT began development of a center for monitoring long distance calls and internet transmissions and other digital information for the exclusive use of the NSA.(Link To Complaint in PDF format). All that might be fine if there had been a court ordered search warrant pursuant to the Fourth Amendment.
82. The center was put into development by ATT following a proposal by the NSA for the construction and development of a network operations center identical to ATT’s own network operations center located in Bedminster, New Jersey for the exclusive use of the NSA.
There wasn't and that is why the plaintiffs sued.
UPDATE: The current Department of Justice leadership is disavowing the Bush II regime's self deceit. It is publishing the deceitful memos that led to torture and illegal spying on Americans (Link to Memos).
We did an article about the veracity of those who used those memo's to present the "I was following orders" defense (Dredd Blog Article).