Though I'm not a big fan, Ted Kennedy hit the nail on the head here:

Think about what we’ve been hearing from the White House in this debate. The President has said that American lives will be sacrificed if Congress does not change FISA. But he has also said that he will veto any FISA bill that does not grant retroactive immunity. No immunity, no new FISA bill. So if we take the President at his word, he is willing to let Americans die to protect the phone companies.