He keeps forcing their hand and putting them in a position where they have to either take enforcement action or give him special treatment they don't give any other public company executives.
This is the law. A CEO cannot say he's secured funding for a buyout if it's not true. A CEO cannot sign an agreement with the SEC then publicly violate it without repercussions.
Potentially market moving tweets get filed all the time. It's not different because it's a Twitter. Here is Dell Technologies filing a few of their deal-related tweets: https://www.sec.gov/Archives/edgar/data/1571996/000119312518...
Carl Icahn literally files single tweets with the SEC. Here are two of his tweets you can find under Apple's SEC filings:
https://www.sec.gov/Archives/edgar/data/320193/0000928464140...
https://www.sec.gov/Archives/edgar/data/320193/0000928464140...
That and Musk runs his company like a child, which allows it to be nimble, but also means every action doesn't get as much scrutiny as they expect.