The proposition is assuming that we know what effect visiting extremist websites will have, we don’t. For some regularly visiting websites that promote violence may end up sickening them and encouraging them to re-evaluate their views rather than further radicalising them. The best way to prevent heinous terrorist acts is not to lock people up on minor offenses but to amass evidence of the much larger offences they are planning and convict them for those offenses rather than a law that will catch many innocents as well as the guilty.