They likely already do this. Somewhere. In a drawer in a file cabinet in a closet with a sign that says beware the cheetah.
http://youtu.be/NLYZn1MXS8s?t=6m57s
The sad part about it all, is that I can imagine what people thought back then when listening to that, especially since he said "by terrorists", which I bet in people's minds it made them forget about all the previous "surveillance on all communications" part of the comment.
Plus, they also must've thought that he "only" means that he will target the communications by some specific terrorists, not that he will spy on everyone to find those terrorists. Most people probably thought he was just referring to the "normal" type of wiretapping certain targets (with a warrant).
That's why Ron Wyden said 2 years ago in Congress that "if the public really knew the secret interpretation of the Patriot Act and FISA, they would be very angry about it", which is exactly what's happening now, because they misled the public about how far this program goes. I don't think most "normal" people actually thought the government is collecting all of their emails, chats, video-talks, and even phone call records.