The liberal democratic argument is that the websites themselves are the ones imposing these harmful effects on their users, therefore by a JS Mill 'harm principle' argument it's fine for the government to intervene and regulate the websites, same justification for regulating, say, the sale of tobacco or knives.
Real harm in this world comes not from these websites, but from government policies that inflict worsening pollution and poverty upon people. The government willingly does nothing about the serious problems, even going in the opposite direction by conspiring with those causing these problems, and instead attacks unsubstantiated pseudo-problems such as what you listed, to appease an uneducated religious minority voter class.
Let's for instance count the number of people that these websites have killed versus the number that guns kill. The counts are about 0 and 40,000+ per year respectively. There is no statistical sense in which the government has their priorities straight.
