The ISPs would not be defining the parameters of what constitutes extremism; they would simply be interpreting the parameters that are given to them by government. They do not need to gauge the extent of the harm from a site simply determine whether it falls within the kind of site they are to be blocking. There would be no expectation that the ISPs would need to work out complex cost-benefit analyses. ISPs, as the purveyors of the internet are perfectly capable of policing their own service, and are well-placed to do so because they manage the software that feeds the internet service. Furthermore, as private agents providing a service, they retain the right to alter the extent of that service if they see fit to do so.