who said they had to be impartial, its a comedy show
See that's just it, it's not. It's an editorialized news program that's funny. It's not a pure comedy show, but this is what comedians who do such programs always use as an excuse "I'm just a comedian." It gives them license to say whatever they want without any sort of repercussions. Bill Maher did it, Al Franken did it, Stewart and Colbert do it, Trevor Noah does it, and now John Oliver does it.
What bothers me is not that it's impartial, it's that it forgoes comedy gold in favor of narrative, preaching, condescension, and pandering. The DNC was an absolute shitshow of comedy potential, from Sarah Silverman being booed and chiding the crowd, a massive delegate walkout, signgate, rioting protesters, lights being shut off, Hillary's "O" face, the Email hacks, Bill Clinton falling asleep during her speech and his balloon chasing, etc.
Instead John Oliver presents a 30 second edit of some of the more eccentric speeches, followed by 15 minutes about Trump. It's an insult to comedy.