The rationale behind regulating Twitter would be the same as the ones behind campaign finance laws: to keep a big corporation from using its power to influence elections. Except the difference is that producing a political movie is clear political speech, while the decision to delete particular items from a firehose of user generated content doesn’t seem to be expressing any message in the part of Twitter itself. As far as I can tell, the Twitterati are not addressing the implications of the “must carry” line of cases like TBS v. FCC which hold that forcing a corporation that provides a pipe for content to carry particular types of content isn’t a first amendment violation.
The Twitter internal emails confirm that folks inside Twitter weren’t treating suppression of the Hunter Biden news as a political statement on the part of Twitter. They were concerned about the potential impact on the election.