Maiden flights are allowed in SAE competitions now because nothing bad has happened yet. The second someone gets hurt by one of those unproven models at an SAE competition, you can rest assured that a rule will be put in place that will require models to be flown successfully prior to the competition.

As for allowing maidens during events, it's a bad idea. "Hey you in the air, we're sending up an airplane that's never been flown before. You might have to run for your life and let your plane crash. Just so you know." Doesn't sound too good, does it?

On top of that, people just don't listen. You can announce until you're blue in the face, and you'll be lucky to get the attention of a fraction of the crowd.