I've seen similar signs, but the ones I've seen were all obviously written and placed to keep you from blocking the entryway. Are you sure about the wording on the sign? It was a pretty narrow doorway with narrow aisles. I can see them putting up the signs for that reason.
(I didn't see the sign you're referring to at this particular show)
Like I said, I dont remember the exact verbage but I was under the impression that they didnt want you doing any private deals on the premises... If they were worried about aisle space they should have outlawed strollers, wheelchairs, and over 38 inch waistlines. (Of which I am rapidly approaching...damnit)