As a slogan, Pete Buttigieg’s health plan looks straightforward: “Medicare for All Who Want It.” Rather than force every American into a government health system, PeteCare would offer a “public option.” Anyone who wants to could buy into the program. Everybody else, Mayor Pete seems to suggest, would be left alone.

But about halfway through his 10-page plan comes an understated line: “Individuals with no coverage will be retroactively enrolled in the public option.” The bottom line is basically what it sounds like: involuntary...