[Jon Stratis]

You answered your own question... "typically" isn't good enough. Having a single team create a mobile hotspot on 5 GHz could be enough to disrupt play. Likewise, having laptops that support 5GHz attempting to create local networks could cause issues as well.

It's generally considered better in situations like this to put a blanket ban out there instead of trusting everyone present to be able to know the difference. Simply removing as many possible points of failure is just good practice, and one of those points of failure is the audiences knowledge of wireless devices.

Of note, At the last off-season event I was at, my new 5 GHz iPhone 5 could detect the field AP, while older devices without 5GHz could not (I looked out of curiosity after reading the Einstein report, though I made no attempts to access the field with it!).