Freedom of speech is important, but must be curtailed when people are threatened. Extremists are a very real threat because their messages and actions galvanize people to take violent, disruptive actions against the state and its citizens. ISPs have a right, and even a responsibility to block extremist websites if it is written into the contract when a user purchases the service. When people opt into an ISP they accept the parameters of the service, so their freedom is not being limited by the blocking of extremist content as they have already accepted that extremist websites are not a part of the access package they bought.