You forget about second order effects once again - HN/reddit facilitate low latency, high scale, high signal communication between millions of people everyday - that's a net positive.

In terms of diet, we tax cigarettes, alcohol and drugs (indirectly) - food that is net negative on society should have their negative externality priced via taxes.

It's pointless banning them (e.g. enforced diets) - see prohibition - leave it legal and tax to cover costs.