Just to explain their thinking, from what I can tell:
These platforms were granted a legal immunity from being sued by users, with the understanding that they're just a tool for users to communicate with each other. Up to now, if somebody said, "I want to sue Twitter for something on their site," the DoJ would have to throw out the case.
So the idea is, let's see if we can put the fear of being the next Gawker into the platforms. At the administration's discretion, they can expose the platforms to trillion dollar lawsuits if they think the platform has gone beyond just policing for offensive content and actually inserted political bias.
And whether there's a lawsuit or not, the EO would forbid the federal government from advertising on a blacklisted 'biased' platform. That's billions of dollars of business at Google, Facebook, Twitter, Reddit, and others
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