Crop insurance critics have a blind spot, seen in the Congressional Budget Office (CBO) report issued last month.

At several points, the CBO asks whether the cost to taxpayers for the current federal crop insurance program is justified compared with the alternative, that is, simply protecting farmers against unusual disasters by providing what it calls “supplemental assistance,” or what used to be less-delicately labeled “ad hoc disaster bailouts.”

The CBO ultimately ducks the question. “It is not possible to know,” the report says, “nor are data...