I'm not a lawyer, but I would imagine one of the big, big differences between news and documentaries is that documentaries are a personal for-profit venture while the news is actually part of the public trust of the airwaves, and is meant to inform citizens (this may be treated different since commercials now make up so much of the news but I don't know if it implies a legal distinction). In other words, it is the tv station's job to report the news in return for being entrusted to the airwaves on which they broadcast, while a documentary filmmaker is making the film in order to push their own career.