[FedCom] Cell Phones ON at POTUS event?

[ed]

a friend sent me this interesting piece of info.  anyone have any idea why 
this was instructed by federal officials?

 >I recently saw the president speak here in Chanhassen, Minnesota (hey,
it was free and I was bored on a saturday)

It was pretty dull, but here's the interesting part. Before the speech
started, they requested that everyone have their cell phones turned ON
during the speech. I'm honestly perplexed as to why this could be.
This obviously confused more people than just myself, as someone else
asked to clarify - indeed, they wanted all of our phones on.