There is a lack of proportionality in punishing users of extremist websites  It is a basic principle of fairness that punishment should fit the crime. [1] In this case the crime is visiting a website, something that in itself may cause no harm at all so why should there be punishment? At best such a law would be punishing on the basis of future harm the accused would otherwise cause if not punished while at worst it would be an arbitrary punishment for people who would never have committed any harm at all.  Not everyone who visits extremist websites is themselves an extremist or is going to be radicalised even after regular visits. Moreover not every person with extremist views is either themselves violent or intending to promote violence. Finally there are a large number of people who regularly visit extremist websites with the purpose of monitoring them; these may be members of the police, the intelligence services, those simply wanting to understand the other and journalists attempting to keep up with extremist trends. What such a law would be doing would be criminalising curiosity.  [1] Hirsch, Andrew Vvon, ‘Proportionality in the Philosophy of Punishment’, Crime and Justice, Vol.16 (1992), pp.55-98.