Perhaps this is a good analogy:
I am a veterinarian. There is a sign in my waiting room requiring clients to keep their pets leashed or caged. Leash laws don't apply indoors, but dogs fight dogs, dogs chase cats, and cats raise all kind of ruckus with each other if my rule is not enforced on my property.
Furthermore, if a dog is threatening, we muzzle it. We use muzzles very infrequently, but I am within my rights to protect myself and my staff when I feel the need to do so.
And clients who want to let their pets run loose or who object to having their dog muzzled, even if it is behaving in a threatening manner, are free to leave. Some have. No problem at my end. The overwhelming majority are understanding.
If everyone was as careful with guns as I am - as the overwhelming majority of you are - additional rules would not be necessary. But we all know that there are plenty of folks who aren't. I therefore respect the fact that gunshow organizers and gunshop owners are looking out for my safety when they post rules, just as my clients understand that, no matter how well behaved their dog is, I can't allow all dogs to run loose in my clinic. The rules and restrictions at gunshows and gunshops might not be perfect, but the proprietors are trying to deal with a difficult situation - a large group of people who vary widely in experience and training and who will, necessarily, be handling firearms - in a way that is both safe and fair to everyone.