They just capture random packets that could have anything in them as they pass by. Those random packets are likely stored on a hard drive along with the GPS location and processed at a later point to try to extract the SSID and whether it is encrypted or not.
So that captured packet could well have “personal” information in it which is what this uproar is about.
Sorry, but a ‘random’ packet would only consist of X number of bytes. Hardly a full e-mail, let alone anything else of ‘value’. At least, from the article, it appears they were war-driving...nothing ‘accidental’ in that regards.