I teach a biennial course called Media & Politics, a catch-all course of the type you find in any half decent political science department in the country. We cover topics like the various theories of how viewers are affected by what they see and read on the news, how structural changes in the media over time have changed the way news is reported, forms of bias, and how political actors try to manipulate the media to their advantage. As far as academic courses go, it's pretty interesting. One thing we cover in depth is the impact of professional norms in journalism on what we see and read. Journalism is no different than any other profession in that there are broadly agreed upon standards of professional conduct (which are flouted on more than a few occasions, of course). One of the most important norms in recent years is objectivity.
Long story short, conservatives have succeeded since the 1970s in beating the "Liberal media" drum so loudly and insistently that journalists are now, as a group, quite defensive about it. Accusing the media of bias is a low-cost and fairly effective strategy. Even if the coverage one receives remains negative, the accusations will at the least lead reporters to pull back a little to prove just how Objective and Fair they are. In the Republican primaries, for example, most of the coverage of people like Trump and Carson is negative. But on the merits it should be a lot more negative. Basically the media should be telling viewers "This is a modern fascist movement driven almost entirely by racism, stupidity, and xenophobia." Since that is a true statement, anything short of that shows that reporters are more interested in being perceived as Fair than in taking any professional risks by inviting the ire of a campaign and its supporters. The path of least resistance is to hold back a bit, play the Objectivity game, and let the campaign pass into the shit heap of history.
It is fair to wonder, though, when a bunch of frothy-mouthed white people literally administer a gangland beatdown to a black protester at a Trump event if treating his campaign like a legitimate political phenomenon is not far more irresponsible than it would be to openly insult and reject it, forsaking all pretense of professional neutrality. There comes a point at which simply covering this without being explicit about what it is abets it. Were I a journalist (a lamentably easy construction to use, as of course I am not) I would have some reservations about what responsibility I might have as a professional in legitimizing that movement. It's obviously difficult to single out just one GOP campaign, and deciding which one is the most openly fascist, racist, and dangerous is like trying to pick the worst L.A. Clippers team from the 1990s. Competition among superlatives is never easy. Nonetheless the Trump campaign is openly embracing email forward / Facebook comments level racism and racist memes at this point. Extraordinary circumstances call for extraordinary responses.
The whole Trump campaign is a live-action internet comment section; nobody feels compelled to take the latter seriously, so why do we have to treat the former with a disingenuous objectivity that it does nothing to deserve?