Both social media companies explained that the moves were aimed at slowing the spread of potentially false information. They failed to give details about how they reached this decision, sparking criticism about the lack of clarity and consistency with which they apply their rules. However, the Republican National Committee believes their choice gave Biden a clear political advantage by keeping bad news out of public view.
The New York Post published a series of stories on Wednesday citing emails, purportedly sent by Biden's son Hunter.
While Facebook outside fact-checkers reviewed the claims, the platform was limiting distribution of the Post's main story. Spokesman Andy Stone explained that Facebook takes this step if it sees "signals" that something gaining traction is false, to give fact-checkers time to evaluate the story before it spreads widely.
Twitter is blocking users from posting pictures of the emails or links to two of the New York Post's stories referring to them, spokesman Trenton Kennedy said, citing its rules against sharing "content obtained through hacking that contains private information."
Twitter went as far as requiring the New York Post to delete its tweet about the story.
Twitter’s policy "prohibits the use of our service to distribute content obtained without authorization" and that it doesn't want to encourage hacking by allowing people to share "possibly illegally obtained materials."
The safety team said in a tweet that the images of emails in the articles "include personal and private information which violate our rules".
Asked for comment about the social networks' actions, New York Post spokeswoman Iva Benson referred NPR to an article by the paper's editorial board.
"Our story explains where the info came from, and a Senate committee now confirms it also received the files from the same source," the editorial stated. "Yet Facebook and Twitter are deliberately trying to keep its users from reading and deciding for themselves what it means."
Twitter and Facebook have been acting more aggressively in recent weeks to curb the spread of false claims and manipulation related to the election, as part of efforts to avoid a repeat of 2016, when Russian-linked actors used social media to target American voters.
Facebook has been warning about the possibility of "hack and leak" operations, where stolen documents or other sensitive materials are strategically leaked — as happened in 2016 with hacked emails from the Democratic National Committee and Hillary Clinton's campaign.
The RNC complained to the Federal Election Commission that Twitter’s blocking of The New York Post articles amounts to an “illegal corporate in-kind political contribution” to former Vice President Biden’s campaign, which is seeking to unseat President Donald Trump, the Republican incumbent.
The companies' moves drew criticism from experts.
Evelyn Douek, a Harvard Law School lecturer who studies the regulation of online speech said, "This story is a microcosm of something that I think we can expect to happen a lot over the next few weeks and, I think, demonstrates why platforms having clear policies that they are prepared to stick to is really important."
"It's really unclear if they have stepped in exceptionally in this case and, if they have, why they've done so," she said. "That inevitably leads to exactly the kind of outcry that we've seen, which is that they're doing it for political reasons and because they're biased."