Jayne,
Usually, such rules come about as a result of some past difficulty. It's easy to imagine the difficulty in this case, in fact, I have personal experience of a similar situation. One time, Kate sent off multiple entries to some state-wide competition. They asked for categories, so she just labeled them according to whatever category seemed to fit. They didn't announce the winners in advance... one was supposed to show up at the dinner. But it was a long drive, and at the last minute something came up... life gets in the way.
Long story short, we won in several categories, and weren't even there to accept the honors (thank goodness, in retrospect: it would have been embarrassing). The whole thing caused some small consternation... and a mutual decision to not send to such contests anymore.
There *is* one rule I'd like to see more widespread: the judge should not know the winner. I remember a chapbook contest a few years back. Kate sent things off, and in the time between sending and the announcement we met the judge at some other event. He later wrote me a nice note, saying he recognized the poems, and had to disqualify me. Would that all judges were so honorable, and had his sense of ethics! The world might be a better place...
Best,
Bill