Showing that something is done purposefully and knowingly instead of negligently and recklessly is hard. It limits the practical enforcement even in the existing false reporting statutes.
Freedom of speech protects strongly from prior restraint (aka pre-publication censorship) and allows people and media to say anything, but there can be legal consequences in many cases. There are the "yelling fire in the crowded movie theater" type cases that lead to convictions when someone has been instigating panic, riots or violence in the social media.
Something like "the migrant caravan" can be safely used as a false news because the news is technically true, it's just exaggerated beyond any reason.
Proper journalism relies on anonymous sources and sometimes journalists are duped when they report something not checked from multiple sources. Legal risk from doing your work would make live unnecessarily difficult.