If I am not mistaken, I believe that open carry states, permit facilities to ban guns on the premises, by putting a sticker on the door that says "no guns allowed". A lot of the anti-gun groups have been putting pressure on companies to ban guns from their premises.
I do agree that it is a little ironic that Republicans who say that the next Supreme Court Justice has to be approved by the NRA, will not have open carry during that convention. It could be quite a spectacle if they did - perhaps Ted Cruz would challenge Donald Trump to a duel (with guns) for the nomination and for the honor of his fair wife.
Seriously, I would hate to think what might happen if guns were allowed. As crazy as it is, I'm old enough to remember when Bobby Kennedy was shot.