Prime Minister Theresa May’s political fortunes may be waning in Britain, but her push to make internet companies police their users’ speech is alive and well. In the aftermath of the recent London attacks, Ms. May called platforms like Google and Facebook breeding grounds for terrorism. She has demanded that they build tools to identify and remove extremist content. Leaders of the Group of 7 countries recently suggested the same thing. Germany wants to fine platforms up to 50 million euros if they don’t quickly take down illegal content. And a European Union draft law would make YouTube and other video hosts responsible for ensuring that users never share violent speech.