1) It is private property. The property owner can do what he wishes.
2) It is ineffectual and only disarms you if you choose to go along with it. If you want to protect yourself, go ahead and carry. No one will frisk you and there are no criminal penalties if you get caught (as far as I am aware - IANAL).
3) The venue owner requires the show promoter to have insurance.
4) The insurance company requires that the promoter have a no loaded guns policy.
5) As a result of 3 and 4 (and morons regularly cranking off rounds at gun shows), the policy isn't going to change.
6) The bored cop at the door doesn't care. He just wants to earn a few extra bucks and get home without getting shot by some moron.
If you really want to get your knickers in a twist over this, have at it; they are your knickers after all. Personally, I choose to spend my time doing something more important, like trying to convince someone on the internet that they are wrong.
And one more thing, using the word confiscate is similar to calling certain guns "assault weapons". It isn't accurate and is used to intentionally invoke an emotional response. It is a rather shabby debating technique.
What is a situation when the word confiscate is appropriate to use along with the word weapon? A fellow is in the midst of an ugly divorce. His wife calls the police and claims he made threats. A judge issues a restraining order. The police show up at his home to
confiscate his weapons. Now that is a confiscation. The police aren't leaving without his weapons, even if they have to send in a SWAT team.