I’m a bit late with this but Stratfor as usual were spot on. After looking back a bit at the NSA’s history, they lay out why PRISM exists and why its existence is so much more problematic than similar developments were in the past (emphasis mine):
The [NSA’s] mission was to stop any further al Qaeda attacks. The means was to break into their communications and read their plans and orders. To find their plans and orders, it was necessary to examine all communications. The anonymity of the Internet and the uncertainties built into its system meant that any message could be one of a tiny handful of messages. Nothing could be ruled out. Everything was suspect. This was reality, not paranoia.
It also meant that the NSA could not exclude the communications of American citizens because some al Qaeda members were citizens. This was an attack on the civil rights of Americans, but it was not an unprecedented attack. During World War II, the United States imposed postal censorship on military personnel, and the FBI intercepted selected letters sent in the United States and from overseas. The government created a system of voluntary media censorship that was less than voluntary in many ways. Most famously, the United States abrogated the civil rights of citizens of Japanese origin by seizing property and transporting them to other locations. Members of pro-German organizations were harassed and arrested even prior to Pearl Harbor. Decades earlier, Abraham Lincoln suspended the writ of habeas corpus during the Civil War, effectively allowing the arrest and isolation of citizens without due process.
There are two major differences between the war on terror and the aforementioned wars. First, there was a declaration of war in World War II. Second, there is a provision in the Constitution that allows the president to suspend habeas corpus in the event of a rebellion. The declaration of war imbues the president with certain powers as commander in chief — as does rebellion. Neither of these conditions was put in place to justify NSA programs such as PRISM.
Moreover, partly because of the constitutional basis of the actions and partly because of the nature of the conflicts, World War II and the Civil War had a clear end, a point at which civil rights had to be restored or a process had to be created for their restoration. No such terminal point exists for the war on terror. As was witnessed at the Boston Marathon — and in many instances over the past several centuries — the ease with which improvised explosive devices can be assembled makes it possible for simple terrorist acts to be carried out cheaply and effectively. Some plots might be detectable by intercepting all communications, but obviously the Boston Marathon attack could not be predicted.