A and ~A are not each evidence for B, if B is "there is a fifth column active". In some ways, as I said, they already knew B - it was true. There were questions of degree - how organized? how ready? how many? - for which A and ~A each provide some hints at.
Perhaps this criticism of the California governor assumes an over-naive probabilistic modelling, with only two events ("no acts of espionage" => "fifth column exists [or not]"). In reality, there existed some non-public information about an existing japanese spy network (MAGIC decodes; informants) that is unlikely to have been mentioned in a public hearing.
Perhaps the reasoning was more like this: "We know that they are already here. We know that some fraction of the population sympathizes with the mother nation. If the fifth column did not exist in an organized form, we might have seen some sabotage already. Since there hasn't been any, maybe they are holding back for a major strike."