That doesn't work when you're talking about data mining. What the NSA does is scan every single call. The filters for this program then yanks out all those to or from certain telephone numbers, and in which certain trigger words or phrases appear. It is a constant filtering process of billions of calls each day. How can you possibly get a warrant for each instance of filtering?
Now once they've pulled those suspicious calls and listened to them, they may then decide to put a direct tap on the phone in the U.S. on which that conversation occured. At that point, they have to comply with the 72 hour warrant requirement. But expecting them to do that for the data mining portion of the program reveals a lack of understanding of what that program is.
Fine. Then let's advocate congress to craft a law saying that the NSA is allowed to do this datamining; if it is so compellingly effective (it may very well be); but it does need to be monitored by some oversight to prevent abuse. Republicans will not be in control of the executive branch forever you know?