That is especially true if our day to day existence includes huge crates that have concealed contents.
Every piece that enters the luggage cargo hold of a plane is examined to save the lives of 300 people; why not give the same care to a show floor that could have upwards of a 1000 people during set-up?
It may be a form of paranoia, or just common sense.

I certainly understand the boost on the local economy, but I have trouble understanding why anyone would choose to exhibit in a trade show in this day & age.
I have exhibited in various shows all over the country since 1991, and stopped in 2002 when I realized that there is absolutely no security in regards to what comes into the building. Yes, there are token guards at the doors, but does anyone really know what is coming in all of those crates? I asked a representative from the drayage company, and the answer was a curt "no". Nobody has any idea what is in any given crate. Use your imagination about what could fit in a 7' tall crate, and how many people are in the convention center during a setup; not a comfortable thought at all.
The article sites 300 exhibitors that chose not to come back - I would wager that the return on investment is not worth the danger to most of them. I would also wager that few shows have actually grown since 9/11.