From a very interesting book, The Cost of Counterterrorism: Power, Politics, and Liberty:

This pattern is a common one. In the aftermath of a terrorist attack, the immediate assumption is that the incident occurred because the state lacked the information and authority necessary to avert it. The executive branch therefore seeks broader powers. And the political stakes are high: legislators are loath to be seen as indifferent to the latest atrocity or, worse, as soft on terror. Accordingly, the legislature grants the executive broader authorities, often under abbreviated procedures and without careful inquiry into what went wrong. Government officials claim that the new powers will be applied only to terrorists. To make the most extreme provisions more palatable, the legislature appends sunset clauses. But in the rush to pass new measures, legislators rarely incorporate sufficient oversight authorities. New powers end up being applied to nonterror- ists – often becoming part of ordinary criminal law. And temporary provisions rarely remain so – instead, they become a baseline on which future measures are built. At each point at which the legislature would otherwise be expected to push back – at the introduction of the measures, at the renewal of the temporary provisions, and in the exercise of oversight – its ability to do so is limited. The judiciary’s role, too, is restricted: constitutional structure and cultural norms narrow the courts’ ability to check the executive at all but the margins.