Late last year, Elon Musk sued nonprofit media watchdog Media Matters for America for defamation. That complaint, filed Nov. 20, alleges that an article by Media Matters showing that X had paired high-profile company ads with pro-Nazi content was somehow contrived, false. By suing, Musk aims to deter Media Matters and other organizations not just from publicizing X’s advertising practices but from fighting disinformation generally. Musk’s suit against Media Matters presents a genuine threat—to the watchdog, of course, but also to the First Amendment itself. In legal parlance, Musk’s suit against MMFA is a textbook SLAPP suit—an intimidation lawsuit, brought...