The no politics rules is mine. Politics discussions (as opposed to discussion about political shows or politicians on shows) always devolve into a shouting match, topic derailer and moderation time suck. This isn't a politics site -- there's lots of places on the internet for that but this isn't one of them. Also it's not black and white either. Certainly some leeway is given in political shows but having a Donald Trump or Hillary Clinton on a TV show isn't an invitation to a broad political discussion about them, the election, the state of the country thanks Obama/Bush, etc etc.
Why politics and not sports, cooking, etc? Simply put: collectively, people don't lose their shit in those other arenas.