Mark Zuckerberg offered a stalwart defense of liberal values on free speech last week, and it’s a sign of our illiberal times that progressives were his biggest critics. A Joe Biden spokesman accused the Facebook CEO of using “the Constitution as a shield for his company’s bottom line,” and pundits on Twitter raged at his refusal to censor ads for Donald Trump.

Mr. Zuckerberg’s offense was standing up for John Mill’s liberal marketplace of ideas that liberals used to stand for. At Georgetown University and in our pages Thursday,...