At the Complexity and Social Networks blog, David Lazer writes about that the data NSA is collecting to investigate terrorist plans could easily slip into nonterrorist criminal investigations and even Hooveresque dirty tricks.
First of all, the data could be used for other criminal investigations. In fact, it is difficult to draw a logical line separating terrorist activity from other criminal acts—e.g., many more people are harmed by other types of crime each year in the US than by terrorism. Obviously, there is a potential for terrorist acts that are especially devastating, and terrorism has particular political significance. But from an empirical/utilitarian calculus, it is difficult to justify why different tools are appropriate for preventing deaths from terrorist activities and not for other criminal activities.
In fact, one would anticipate that for people who “live on the grid” in the US, the data produced could be very powerful for investigations. Consider simply the potential use of locational information from cellular phones, where one might be able to identify who was near the crime scene at the time of the crime. Even if the perpetrator of the crime was sensible enough to turn off their cell phone (and many would not), this would be an effective way to identify material witnesses.
Second, there is the possibility that these data could be used for political purposes. Consider the value of tracking the communication and location of the news media and political opponents. One could see which opponents were talking with each other, which of your erstwhile allies were talking with opponents. If one suspect an opponent was having an affair, you could correlate their locations and communications with those of other people, etc.
In a political culture which is increasingly polarized, where the “other side” is increasingly demonized, it is plausible that such tactics in the future could be rationalized by those in power, if they felt that there was a sufficiently low probability of being caught.
That's why we have statutes that define how such information should be used. What's wrong with the Administration's current policy isn't that it's wrong to track down terrorist phone calls. It's that it's wrong for the executive branch to decide to break the law rather than seeking to change it. The fact that they didn't want anyone to know they broke it strongly suggests data creep is not an unintended consequence.