There is a big diffrence between a car, knife and a gun.
Take the keys out of a car and the chances of it hurting anyone go way down.
A kife is a contact weapon, you can't say kill some one from across the convention floor with it.
lets just play the what if game. lets just say I'm nuts, I go to this convention where I can freely pick up a gun, there is no zip tie, no teather, no modifcation to the gun in anyway. If I were to grab a semi auto rifle, say a ar or ak, and I have a loaded mag on my person. What do you think the chances of some one stoping me from loading the gun, and firing all 30 shots into the crowd?
And before you say this type of thing does not happen. It damn well has happen. Not the NRA convention mind you, but other places.
I know I've told all of you about the hardware store that use to sell guns when I was a kid by my house. How a guy stole a bullet, asked to see a 44 and blew his brains out. now granted he didn't go shooting into a crowd, but he could just have easliy turned it on the shop owner and killed him.