a) because services like Twitter and Facebook have to show something in the timeline. What they show is this job b) because in practice, an approach too laisse

a) because services like Twitter and Facebook have to show something in the timeline. What they show is this job b) because in practice, an approach too laissez-faire here has negative impact both on customer adoption (most people don't actually enjoy cesspools, and they leave when they decide a service is one) and regulation (even in countries with broad free-speech protections, some communication is de facto illegal and there are consequences for a service taking no responsibility for keeping their house in order).