As it’s at the DuPage County Fairgrounds, I strongly suspect some gubbermint weasel insisted that they do this as the price of retaining their event permit.
You sir, are completely correct. I live 1.5 miles from those fairgrounds. Have been going to this show for 45 years. But I boycotted attending last weekend because of the "ban". The renewal of the contract for the show with the fair grounds association is next year, and this sportsman's group is "walking on egg shells".
I talked to a number of friends who did attend and set up tables for things to sell at the show. They allowed AR-15 lowers, At-15 uppers, etc. But no complete guns. Its all Kabuki.