But then you can easily, heck trivially, argue that people should be forced to exercise or to eat a particular diet that makes them more productive. Look, more goods and services are rendered! It's a public situation!
People should be forced to live within 5 miles of work, because they're not being productive during their commute. Also, HackerNews and reddit should be forcibly taken down because of their contribution to idleness at work.
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.
People should be forced to live within 5 miles of work, because they're not being productive during their commute. Also, HackerNews and reddit should be forcibly taken down because of their contribution to idleness at work.