Presumably in some of those cases, the argument was that if they could scan a large number of peoples email for keywords they might have been able to find some

Presumably in some of those cases, the argument was that if they could scan a large number of peoples email for keywords they might have been able to find some evidence as to who was involved. You generally can't get a warrant to scan a large number of peoples email. Especially if that large number amounts to everyone. I don't agree that they should have the ability to do this, even if it saves lives. Because handing over this sort of power to a government only leads to the creation of a police state, which ultimately will probably ruin more lives. Freedom costs lives.