The John Doe investigation of Wisconsin conservatives collapsed last week with a powerful decision from the Wisconsin Supreme Court that called state prosecutors’ theory of campaign-finance law “unconstitutional” and “unsupported in either reason or law.” But the legal exoneration shouldn’t pass without noting the hardship the secret probe imposed on its targets and on political debate in Wisconsin.

For the past few days, I’ve been talking to the targets of the task force of Milwaukee Democratic prosecutors, the Wisconsin...