Media outlets noticed there is a lot of spam and misinformation happening on social media, and saw not only great headlines to garner clicks, but also an opportunity to make people distrust smaller sources of information in favor or big, established brands and companies. They drove subscriptions up by creating an aura of mistrust and impending doom [2][3] after historical decline due to the internet.
Did the inflammatory posts actually affect the election? It's unclear, and if so it was likely less so than the coverage of traditional media outlets [4]. That being said, there are inflammatory parties on the internet, and since Americans now seem to think the future of their country depends on removing them, Facebook needs to abide for the sake of PR. Note: this actually costs Facebook time and money (they are hiring a lot of people for this effect [5]) but they've been bashed so much on the media they need all the PR stunts they can get.
It is a slippery slope, I agree. But it's the one people in the US want to follow, again, because they are scared. Fearful people take slippery slopes and focus on apparent short-term solutions instead of long-term objectives (fixing the American political system so the population actually votes for who is president would be a good place to start). There's that quote from the Star Wars trilogy (the best line in the prequels, IMO): "So this is how liberty dies...with thunderous applause." This comes to mind here. People will willfully act against their interests out of fear.
[1]: https://www.channel4.com/news/factcheck/americas-long-histor...
[2]: https://money.cnn.com/2016/11/17/media/nytimes-subscription-...
[3]: https://money.cnn.com/2017/09/26/media/washington-post-digit...
[4]: https://fivethirtyeight.com/features/how-much-did-russian-in...
[5]: https://www.cnbc.com/2017/10/31/facebook-senate-testimony-do...