The song if what I was told years ago is true originates from the days of the press gang in England, where young men were often "pressed" into service in the royal navy.
The favourite place for this to happen would be taverns in coastal towns, and on many occasions this would occur during fairs and markets as it gave the best recruiting chances.
The locals would mainly sit watching the door to give them a chance to escape should the press gang arrive, while the green country youngsters would enjoy too much ale and be off guard.

Hence Johnny probably either didn't come back from the fair at all or came back several years later...