Just this weekend the mainstream media highlighted three ways that its unwritten rules actually encourages extreme and non-normative political behavior.
1) If the mainstream media sees details of a policy, it will tear it apart. But, if it does not see details of a policy, it will take its supporter's word for what it does, even if it is impossible and otherwise absurd in the context of the supporter. Example from this weekend was the Green New Deal, which can be read here. I fully support some of this and have serious concerns with some of it. It is ambitious and necessary, even if it needs to evolve to work. But, the mainstream media spent most of the weekend focused on the most extreme aspects of the deal or some side debate over talking points that may or may not have been on a website. Seeing the details should signal the seriousness of the effort, but the mainstream media gets bogged down in the negative interpretation or salaciousness of details. Conversely, Republicans spent eight years claiming to have a magical healthcare plan to replace ObamaCare. It would cut costs, raise coverage, and there would be a unicorn in every driveway. Of course, they never showed anyone this magical healthcare plan, so the mainstream media just reported on what the Republicans said the plan would do. Thus, Americans were legitimately surprised when Republicans took control of House, Senate, and President and had absolutely no plan for replace ObamaCare.
This policy encourages non-normative behavior: better to have no plan get favorable press than to have a plan so the press can tear it apart. And, this policy encourages extreme behavior: no need to moderate your position if you can have extreme policies, but mainstream media will report whatever claim you are doing, not what you are doing.
2) The mainstream media holds Democrats to a higher standard than Republicans, because they know Republican leadership does not care about integrity or ethics. This asymmetric accountability is absurd, and allows Republicans to become more extreme and cruel without a necessary check. This weekend Republican President Trump mocked the genocide of Native Americans, then his son joined in with even more direct commentary. A Republican leader, communications director for Turning Point USA, glorified Hitler's early work (she is totally against his post-September 1, 1939 expansionist policies). There is justifiable coverage of two very credible claims of sexual assault on the lieutenant governor of Virginia, but no one bothers to mention the 19+ credible claims against the Republican president. Mainstream media does not bother to report on Republican ethics, because they know Republican leaders will not take action. But, they will never take action if the mainstream media continues to give them a free-pass.
This policy encourages non-normative behavior and extreme behavior: Republicans are free to lack ethics and integrity because mainstream media is not interested in reporting on them.
3) Mainstream media still does not know to report on President Trump, they cannot help but drag the target of his lies or harassment into their coverage. Giving him a win, and incentivizing lies and harassment. This weekend he went after Senator Warren by degrading Native Americans. Why report the target? Just report on the racism and racist policies, without providing that context. There is a deep academic literature that shows that reporting a lie, even with clear context that it is a lie, just spreads it further. President Trump rose to the top of the Republican Party by leading the charge of racist comments against President Obama. The media repeated this false accusations ad-nauseum, helping to make them mainstream. They could have just reported that Donald Trump is making racist lies. It is that simple.
This policy encourages non-normative behavior: mainstream media rewards lying and harassment by repeating it over and over.