I actually read up on that a bit maybe a year ago. Bobbie Gentry revealed in an interview that it's undetermined what Billy Joe threw off the bridge. Not germane to the theme of the song. She said the indifference of the family sitting at the table during the meal regarding Billy Joe's suicide was the key point.
I realize you likely wrote this question in jest, but a year ago or so when I was intently listening to this song, I wondered what the point of the song was. Side note: I lived for a couple of years in the Mississippi delta area and have an inkling of what this (likely) sharecropper family experienced, beautifully expressed by Bobbie in her music and lyrics.
