Over the years, these things have become necessary due to the stupid stunts people pull. Several years back, a friend of mine was working a table at a show when
BANG and he felt something impact the side of his pants near his ankle. Some dummy had come to the show looking for ammo, had either removed the zip tie or smuggled it in, then found ammo that he thought would "fit." He chambered it and, in practically the same motion pulled the trigger. Unfortunately, it was the correct ammunition. The impact my friend felt was splinters from a plywood box that he used to carry in his stuff. Missed his leg by inches. The guy was charged with discharging a firearm within city limits and the dealer was ejected from the show (probably for not watching his table close enough).
Another example. An LE buddy of mine decided he wanted to get is off-duty Glock worked on by an armorer at the show that we both knew. Walks right into the show with the gun concealed. Talks to the armorer about changing something, I forget what. He simply lifted his shirt and pulled the gun out of its holster. Anyway, for the said modification, the gun needs to be disassembled. The armorer dropped the mag and cleared the gun before starting and out popped a .45 Hydrashok. I had assumed my friend had either unloaded before leaving the car or decided against the mod at the last minute. I couldn't believe that he had handed the armorer a loaded weapon.
Disaster averted by the armorer's strict adherence to safety rules.
Personally, I don't think anybody short of uniformed, on-duty LE should be allowed to enter these shows with loaded guns. But I do believe that the show promoters should provide a station for people legally carrying weapons to safely unload before entering and load up before leaving. Something that will absorb a negligent discharged.
Is it illegal to ignore the signs and carry anyway? Depends upon the state. In some states, it can be serious. Most times in VA, they tell you to get lost if you're caught (I've seen it happen a few times). Recently, I have seen a couple guys get written summons for tresspassing at local shows. Some promoters are now placing signs that violators will be prosecuted to the extent of the law, which probably means an unpleasant talk with city/county police and charged with tresspassing.